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Accession  No.       92351    .   ClaxsNo. 


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WORKS  OF  JAMES  H.  FUERTES 

PUBLISHED    BY 

JOHN    WILEY    &    SONS. 

Water  and  Public  Health. 

The  Relative  Purity  of  Waters  from  Different 
Sources.  iamo.  x  -f-  7^  pages.  68  figures,  cloth, 
$,.50. 

Water  Filtration  Works. 

iznao,  xviii  4-281  pages,  45  figures  and  20  half-tone 
plates,  cloth.  $2.50. 


CLEANING  THE  SEDIMENTATION  BASINS  AT  ST.   Louis,  Mo. 

Frontispiece. 


WATER  FILTRATION  WORKS. 


BY 


JAMES    H.    FUERTES, 

Member  of  the  American  Society  of  Civil  Engineers. 


FIRST   EDITION. 
FIRST     THOUSAND. 


NEW  YORK: 

JOHN   WILEY  &   SONS. 

LONDON:  CHAPMAN  &   HALL,  LIMITED. 

1901. 


Copyright,  1901, 

BY 
JAMES   H.  FUERTES. 


ROBERT  DRUMMONr,   PRINTER,    *K\V  YORK. 


PREFACE. 


IN  1839  James  Simpson  built  a  set  of  filters  for 
the  Chelsea  Water  Company  at  London.  These 
filters,  the  first  constructed  for  the  purification  of  a 
municipal  water-supply,  were  intended  merely  to 
clarify  the  water,  no  investigations,  at  that  date, 
having  been  made  to  determine  what  results,  other 
than  clarification,  could  be  obtained  by  filters  prop- 
erly designed  and  operated.  Twenty-seven  years  later 
Mr.  James  P.  Kirkwood  visited  Europe  for  the  city 
of  St.  Louis  for  the  purpose  of  studying  the  methods 
of  filtration  in  use  abroad.  On  his  return  he  sub- 
mitted a  report  in  which  the  filtering  of  the  St.  Louis 
water  was  recommended,  following  this  report  in  1869 
with  a  treatise  on  the  "  Filtration  of  River  Waters." 
This  book  contained  the  results  of  his  observations 
and  studies  of  thirteen  filter-plants  in  Europe,  and  was 
the  first  publication  to  appear  in  English  on  the  sub- 
ject of  filtration.  Able  as  was  this  discussion,  how- 
ever, public  interest  in  the  question  of  the  purification 
of  polluted  waters  remained  dormant  in  the  United 

92351 


VI  PREFACE. 

States  until  the  results  of  the  valuable  experimental 
work  conducted  at  the  Lawrence  Experiment  Station 
were  published  in  the  Annual  Reports  of  the  Mas- 
sachusetts State  Board  of  Health.  These  reports 
attracted  world-wide  attention,  cleared  up  many 
points  that  had  been  but  imperfectly  understood  in 
the  phenomena  attending  filtration,  and  gave  a  stimu- 
lus to  public  interest  which  has  resulted  in  the  estab- 
lishment of  many  filter-plants  in  the  United  States,  as 
well  as  in  other  countries.  With  the  exception  of 
the  above-named  works,  and  the  authoritative  and 
useful  book  published  in  1895  by  Allen  Hazen,  en- 
titled "The  Filtration  of  Public  Water-supplies," 
the  most  valuable  data  bearing  upon  the  subject  of 
filtration  are  to  be  found  in  the  not  generally  accessible 
reports  of  special  investigations,  in  the  current  techni- 
cal journals  in  the  United  States  and  Europe,  and  in 
a  few  works  on  the  purification  of  water,  which  treat 
the  subject  principally  from  the  sanitary  and  chemical 
points  of  view.  The  author  has  drawn  freely  upon 
these  sources  of  information,  particularly  upon  the 
valuable  Annual  Reports  of  the  Massachusetts  State 
Board  of  Health,  and  the  reports  on  the  purification 
of  the  Washington,  Pittsburgh,  Cincinnati,  Louisville, 
and  Providence  water-supplies.  Persons  familiar  with 
the  reports  of  the  Massachusetts  State  Board  of  Health 
will  recognize  in  the  first  pages  of  the  chapter  on  The 
Theory  of  Slow  Sand-filtration  the  substance  of  the 
very  clear  statement  of  the  phenomena  attending 
decay  and  regeneration  written  by  Dr.  Thomas  M. 
Drown  to  whom  full  acknowledgment  is  tendered. 


PREFA  CE.  VH 

Among  his  professional  colleagues  who  afforded 
him  opportunities  of  visiting  the  plants  under  their 
direction  and  also  furnished  him  with  many  valuable 
data  concerning  the  construction  and  operation  of 
filtration  works,  the  author  wishes  specially  to  men- 
tion the  late  F.  Andreas  Meyer,  City  Engineer  of 
Hamburg;  Wm.  H.  Lindley,  Civil  Engineer,  Frank- 
fort-on-the-Main,  Germany;  M.  Peter,  City  Engineer, 
and  M.  Bertschinger,  City  Chemist,  Zurich;  Director 
Beer  and  Superintendent  Engineer  Anklamm,  Berlin, 
and  Wm.  Anderson,  Treasurer  and  General  Manager 
of  the  Edinburgh  Water-works. 

The  author  is  also  under  obligations  to  Mr.  William 
Wheeler,  Consulting  Engineer,  Boston,  for  the  photo- 
graphs of  the  Ashland,  Wis.,  and  Somersworth,  N.  H., 
covered  filters;  to  Mr.  Geo.  I.  Bailey,  Superintendent 
Bureau  of  Water,  Albany,  for  valuable  data  and  for 
the  photographs  of  the  Albany  filters;  to  Mr.  Edward 
Flad,  Water  Commissioner,  St.  Louis,  Mo.,  for  the 
pictures  of  the  Intake  and  Settling  Basins  of  the  St. 
Louis  Water-works ;  to  Mr.  Morris  Knowles,  Assistant 
Engineer  in  charge  of  the  Testing  Station,  Philadel- 
phia, for  the  photographs  reproduced  in  Plates  XI, 
XVI,  XVII,  XVIII  and  XIX;  and  to  the  New  York 
Continental  Jewell  Filtration  Company  for  valuable 
data,  and  for  the  illustrations  forming  Plate  XV  and 
Figs.  37  to  45  inclusive. 

During  the  past  decade  great  advances  have  been 
made  in  the  development  of  processes  for  the  purifi- 
cation of  polluted  waters.  These  processes,  and  the 
works  necessary  for  carrying  them  out,  are  described 


VI 11  PREFACE. 

with  sufficient  fulness  in  the  following  pages  to  indi- 
cate the  results  that  may  be  attained  in  the  matter  of 
the  purification  of  polluted  waters,  the  means  of  at- 
taining these  results,  and  the  elements  entering  into 
the  design,  as  well  as  into  the  cost  of  the  necessary 
works,  both  as  regards  construction  and  operation. 

JAMES  H.  FUERTES. 

NEW  YORK,  April,  1901. 


TABLE  OF  CONTENTS. 


CHAPTER  I. 

INTRODUCTORY. 

PACE 

Water  and  Public  Health I 

Typhoid  Fever  and  Water-supply i 

The  purification  of  Water  by  Natural  Agencies 3 

Effects  of  Aeration 4 

Effects  of  Storage  on  the  Quality  of  the  Water 5 

Effects  of  Mud  Deposits  in  Reservoirs  on  the  Quality  of 

the  Water 6 

Effects  of  the  Fouling  of  Water-mains  on  the  Quality  of 

the  Water 9 

Self-purification  of  Streams 12 

Effect  of  the  Freezing  of  Water  on  its  Quality. 12 

The  Protection  of  Water-supplies 13 

Permissible  Pollution 13 

Legal  Protection  of  Water-supplies 13 

Provisions  for  Betterment  of  Water-supplies 14 

Requirements  of  Water  Companies  in  the  Matter  of  Pro- 
tection    15 

Protection  of  Surface  Supplies 15 

Ownership  vs.  Legal  Protection 17 

Effects  of  Surface  Washings 17 

Protection  from  Sewage  Pollution 22 

Protection  of  Lake  Supplies 23 

The  Purification  of  Water  by  Filtration 25 

ix 


X  TABLE  Of  CONTENTS. 

CHAPTER  II. 

INTAKES,    SEDIMENTATION,    AND    SETTLING   BASINS. 

PAGE 

Intakes 28 

Tidal  Streams 28 

Rivers  with  Stable  Banks  above  Flood  Height,  and  with 

small  Range  of  Fluctuation  of  Level 29 

Rivers  with  Stable  Banks  below  Flood  Height 30 

Rivers   with    Shifting    Banks   and   Bottoms,    and    Great 

Range  of  Fluctuation  of  Level 33 

Sedimentation 33 

Amount,  Character,  and  Distribution  of  Sediment 33 

Turbidity 36 

Standards  of  Measurement 36 

Rate  of  Sedimentation 37 

Effects  of  Winds , 39 

Effects  of  Temperature 40 

Effects  of  Light 4° 

Use  of  Chemicals  to  Aid  Sedimentation 41 

Results  to  be  Obtained  by  Sedimentation 41 

Efficiency  of  Sedimentation 42 

Settling  Basins 45 

Designing 45 

Location 45 

Capacity 46 

Depth 47 

Length , 48 

Velocity  of  Flow  Through  Basins 48 

Form  of  Basins 5* 

Arrangements  to  Draw  Off  Water  Longest  in  Storage  51 

Locations  of  Inlets  and  Outlets 52 

Construction 53 

Bottoms 53 

Underdrainage 55 

Sides 55 

Regulating  Apparatus 5^ 

Removal  of  Sediment 60 

Roofing 61 

Cost..                                                            62 


7 'ABLE   OF  CONTENTS.  XI 


Operation 63 

Rate  of  Flow  Through  Settling  Basins 63 

Amount  of  Sediment  to  be  Expected   64 

Depth  of  Sediment  to  be  Provided  for 66 

Periods  of  Cleaning t . . .  66 

Amount  of  Water  Necessary  for  Cleaning 67 

Methods  of  Cleaning 71 

Cost  of  Removing  Sediment 72 

Relative  Advantages  of  the  Fill-and-Draw   and   the 

Continuous  Methods  of  Operation 73 


CHAPTER  III. 

THE    PURIFICATION    OF    WATER    BY    SLOW   SAND-FILTRATION. 

Introduction 75 

Types  of  Filters  Used  for  Municipal  Supplies 75 

Slow  Sand-filtration 76 

Rapid  Sand-filtration 76 

Theory  of  Slow  Sand-filtration 77 

Action  of  Slow  Sand-filters 80 

Bacterial  Efficiency 81 

Bacterial  Purification 81 

Hygienic  Efficiency 81 

Influence  of  Character  of  Water 82 

Influence  of  Size  and  Character  of  Sand 84 

Influence  of  Compacting  of  Sand  Layer 84 

Influence  of  Depth  of  Sand  Layer 86 

Influence  of  Loss  of  Head 90 

Influence  of  Depth  of  Water  on  Filter  Surface 93 

Influence  of  Rate  of  Filtration 93 

Influence  of  Sudden  Change  of  Rate 95 

Influence  of  Age  of  Filters 96 

Influence  of  Scraping 97 

Influence  of  Method  of  Application   of  Water  to   Inter- 
mittent Filters 99 

Influence  of  Method  of  Putting  Filters  in  Service  after 

Scraping 100 


Xll  TABLE  OF  CONTENTS. 


PAGE 

Influence  of  Temperature roc 

Conclusions , 101 


CHAPTER  IV. 

THE  DESIGN,  CONSTRUCTION  AND  OPERATION  OF  SLOW  SAND-FILTERS. 

Designing 103 

Per  Capita  Water  Consumption  and  Waste  Reduction.. . .  103 

Number  of  Filter  beds  Required 107 

Excess  Area  Required 107 

Location  and  Grouping  of  Beds 113 

Shape  of  Beds 115 

Depth  of  Beds 117 

Construction -. 117 

Preparation  of  Site 117 

Side  Slopes  and  Bottoms 117 

Precautions  to-  Prevent   Water    Passing   to  the    Under- 

drains  in  an  Unfiltered  State 119 

Effects  of  Sun  on  Paving  of  Open  Beds 119 

Covered  vs.  Uncovered   Filters 120 

Drainage  of  Roofs 130 

Ventilation 133 

Tramways  for  Sand  Haulage 134 

Bottoms  and  Forms  of  Same 134 

Underdrains 139 

Gravel  Layers 142 

Filter-sand 146 

Depth 149 

Character  of  Sand 150 

Placing  Sand 153 

Placing  Gravel 153 

Structural  Details 154 

Sand  Washing 154 

Regulating  Apparatus 161 

Cost  of  Slow  Sand-filters 174 

Operation 179 

Relative  Locations  of  Filters  and  Filtered-water   Reser- 
voirs  v , .  v 1 79 


TABLE   OF  CONTENTS.  Xlll 

PAGB 

Scraping  Filter-beds 180 

Cost  of  Scraping 183 

Frequency  of  Scraping 185 

Effects  of  Covers  on  Frequency  of  Scraping 187 

Transporting  Sand  to  Washers 190 

Cost  of  Sand  Washing 190 

Quantity  of  Water  Used  for  Sand  Washing 197 

Loss  of  Sand  in  Washing 198 

Ice  on  Open  Filters 198 

Refilling  Filters  after  Scraping 199 

Double  Filtration 200 


CHAPTER    V. 

THE    PURIFICATION   OF   WATER   BY    RAPID    SAND-FILTRATION. 

Theory  of  Rapid  Sand-filtration 201 

The  Coagulant  and  its  Effect  on  Efficiency  of  Filtration..  201 

Quantity  of  Coagulant  Required * 203 

Time  of  Admixture  of  Chemical 207 

Effect  of  Filtering  Medium 210 

Effect  of  Rate  of  Filtration 211 

Effect  of  Loss  of  Head 213 

Effect  of  Washing  Filters 214 

Effect  of  Trailing 215 

CHAPTER   VI. 

THE    CONSTRUCTION    AND    OPERATION    OF    RAPID    SAND-FILTERS. 

Gravity  and  Pressure  Filters 217 

Introduction  of  Chemical  Solution 225 

Regulating  Apparatus 231 

Washing  Arrangements 234 

Cost  of  Rapid  Sand-filters 238 

Operating  Rapid  Sand-filters 242 

Period  of  Time  Between  Washings 242 

S,UH!  . 243 


XIV  TABLE   OF  CONTENTS. 

PAGE 

Labor  for  Operating  Rapid  Sand-filters 243 

Quantity  of  Water  Required  for  Washing  and  Rate  of  Appli- 
cation of  Wash-water. 244 

Wasting  Wash-water 245 


CHAPTER   VII. 

CONCLUSIONS. 

General 246 

Combinations  of  Rapid  and  Slow  Sand-filters 247 

The  Anderson  Process 249 

The  Pasteur-Chamberland  Process 250 

The  Fischer  or  Worms  Process 250 

The  Maignen  Process 255 


CHAPTER   VIII. 

FILTERED-WATER    RESERVOIRS. 

Location 256 

Shape 256 

Circulation 257 

Capacity 257 

Depth 260 

Effect  of  Pumping  on  Depth 261 

Bottoms 262 

Workmanship 263 

Walls 265 

Covers 266 

Ventilation 267 


LIST  OF  FIGURES. 


FIG.  PAGE 

1.  Rate  of  Subsidence  of  Mississippi  River  Water  at  St.  Louis, 

Mo 38 

2.  Rate  of  Clarification  of  Mississippi  River  Water  at  St.  Louis, 

Mo.,  in  Passing  Slowly  Through  a  Long  Flume 44 

3.  Effect  of   Size  of   Sand  Grain  on  Efficiency  of  Slow  Sand- 

filtration 86 

4.  Diagram  Showing  Retention  of  Bacteria  and  Nitrogen  in  Ten 

Slow  Sand-filters,  at  the  Lawrence  Experiment  Station  ...     88 

5.  Arrangement  of  the  Lake  Mueggel  Filter-plant,  Berlin,  Ger- 

many   114 

6.  Plan  of  Berlin  (Mueggel)  Filter-bed 116 

7.  Groined  Arches 122 

8.  Masonry  Groined  Arches  with  Arch  Ribs 123 

9.  Concrete  Groined  Arches 124 

10.  Domed  Covering  with  Arch  Ribs 127 

11.  Cylindrical  Arches 128 

12.  Flat  Domes 129 

13.  Concrete  Darned  Construction 130 

14.  Typical  Plan  and  Sections  of  Covered  Slow  Sand-filter 137 

15.  Plan  of  Filter-bed,  Zurich,  Switzerland 138 

1 6.  Conversion  Diagram.     Gallons  per  Day  into  Cubic  Feet  per 

Second  and  per  Minute,  and  Gallons  per  Second 140 

17.  Conversion  Diagram.    Million  Gallons  per  Acre  into  Gallons 

per  Square  Yard,  Gallons  per  Square  Foot,  and  Vertical 
Depth  in  Feet 141 

18.  Conversion  Diagram.     Million  Gallons  per  Acre  per  Day  for 

Different  Areas  into  Cubic  Feet  per  Second 142 

19.  Diagram  Showing  Frictional  Loss  of  Head  in  Pipes 143 

20.  Hollow  Floor,  Zurich  Filters 144 

xv 


XVI  LIST  OF  FIGURES. 

FIG.  PAGE 

21.  Diagram   Showing   Head   of   Water   Consumed   in    Passing 

Horizontally  Through  Gravel  Layers 145 

22.  Cross-section  T*hrough  Ejector  Sand-washer 158 

23.  Plan  of  Ejector  Sand-washer 158 

24.  Longitudinal  Section  Through  Ejector  Sand- washer 158 

25.  Regulating  Apparatus  in  Use  at  Yokohama,  Japan 162 

26.  Regulator  in  Use  at  Koenigsberg,  Germany 163 

27.  Regulator  Designed  by  Henry  C.  Gill,  and  Used  at  the  Berlin 

Filter-plants 1 64 

28.  Regulator  Recommended  by  James  P.  Kirkwood  for  St.  Louis  166 

29.  Regulating  Apparatus  in  Use  at  Hamburg,  Germany 166 

30.  Regulating   Apparatus    Designed   by  Allen    Hazen   for   the 

Albany  Filters 167 

31.  Regulator  in  Use  in  Zurich,  Switzerland 169 

32.  Regulator   Designed  by  Wm.   H.  Lindley  for  the  Filters  at 

Warsaw 169 

33.  Regulator  Suggested  by  the  Mayor's  Expert  Water  Commis- 

sion, Philadelphia 170 

34.  Regulator  Designed  by  the  Author  for  the  Tome   Institute 

Filters 171 

35.  Regulator  in  Use  at  Worms,  Germany. .  c 1 73 

36.  Regulator  in  Use  at  Tokio  and  Osaka,  Japan 173 

37.  Sectional  View  of  Jewell  Subsidence  Gravity  Filter 220 

38.  Plan  of  Continental  Gravity  Filter 223 

39.  Sectional  Elevation  of  Continental  Gravity  Filter 224 

40.  New  York  Sectional-wash  Gravity  Filter 225 

41.  New  York  Sectional-wash  Pressure  Filter 226 

42.  Typical   Chemical   Solution    Measuring   Tank    for    Gravity 

Filter 227 

43.  Chemical  Solution  Measuring  Tank  for  Pressure  Filter 228 

44.  Chemical   Solution    Pump    for    Either  Gravity  or  Pressure 

Filters 230 

45.  Weston's  Automatic  Controller  for  Rapid  Sand-filters 231 


LIST  OF  PLATES. 


PAGE 

Frontispiece.  Cleaning  the  Sedimentation  Basins  at  St.  Louis,  Mo. 
I.   Removal  of  30   years'  accumulations   of  mud  from   the 

Mt.  Airy  Reservoir,  Philadelphia 7 

II.  Intake  of  St.  Louis,  Mo.,  Water-works 31 

III.  Settling  Basin,    Albany,  N.  Y.    View   showing  aerating 

inlets  for  raw  water,  slope  paving,  concrete  bottom, 
and  method  of  removing  sediment 57 

IV.  Albany  Filtration  Plant.  General  view  of  Settling  Basin, 

showing  removal  of  sediment  deposited  from  the  water    69 
V.  Interior  view  of  Ashland,  Wis.,  Covered  Filter.     First 
adaptation  in  the  U.  S.  of  the  groined  arch  for  filter- 
covers 125 

VI.  Somers worth,  N.  H.,  Covered  Filters.    Birdseye  view  of 

centering  for  groined  arches 131 

VII.  Somersworth,  N.  H.,  Covered  Filters.     View  taken  dur- 
ing construction 135 

VIII.  Somersworth,  N.   H.,  Covered   Filters.     View  showing 

the  underdrains  and  gravel  being  placed  in  position...  147 
IX.  Interior  view  of  Ashland,  Wis.,  Covered  Filters,  taken 

when  the  filtering  sand  was  being  placed  in  position...  151 
X.  Interior  view  of  Somersworth,  N.  H.,  Covered  Filters, 
showing  the  filtering  sand  being  placed  in  position  in 
three  layers,  the  underdrains,  and  gravel  surrounding 

them 155 

XI.  Method  of  scraping  slow  sand-filters 181 

XII.  Albany  Filtration  Plant     Wheeling   out   sand  removed 

from  filter  after  scraping  igt 

xvii 


xviil  LIST  OF  PLATES.; 

PAGE 

XIII.  Albany    Filtration   Plant.     Sand-washers  as  originally 

built.     The  dirty  sand  was  wheeled  in  barrows  to  the 
washers 193 

XIV.  Albany  Filtration  Plant.     Improvement  in  sand-washing 

machinery.     The  dirty  sand  is  conveyed  to  the  wash- 
ers through  a  pipe  by  a  portable  ejector-hopper  and  a 

stream  of  water 195 

XV.  Interior  view  of  East  Albany  Filter  Plant 221 

XVI.  Agitator,  Jewell  Filter 235 

XVII.  Agitator,  Warren  Filter 239 

XVIII.  Worms  or  Fischer  Plate  ready  to  be  placed  in  filter 251 

XIX.  Broken  Worms  or  Fischer  Plate,  showing  interior  cavity  253 


WATER  FILTRATION  WORKS. 


CHAPTER  I. 

INTRODUCTORY. 

WATER   AND    PUBLIC    HEALTH. 

Typhoid  Fever  and  Water-supply. — The  purity  of  wa- 
ter depends  upon  its  source  and  upon  the  polluting 
and  purifying1  influences  to  which  it  has  been  sub- 
jected. It  is  now  well  known  that  in  a  community 
using  water  polluted  with  sewage  the  general  health 
tone  gradually  falls  lower  and  lower  and  its  death  rate 
increases  proportionately.  Among  the  diseases 
known  to  be  capable  of  transmission  by  drinking-wa- 
ter, typhoid  fever  holds  a  prominent  position.  As  it 
is  nearly  always  present  in  cities,  its  continued  preva- 
lence, in  abnormal  proportions,  indicates  excessive 
pollution,  by  sewage  or  fecal  matter,  of  the  drinking- 
water  supplied  to  the  community.  This  is  well  set 
forth  in  Table  I,  in  which  the  death  rates  are  the 
averages  for  several  years: 


WATER  FILTRATION   WORKS. 
TABLE   I. 


Kind  of 
Water  Used. 

City. 

Source  of  Supply. 

Typhoid-fever 
Death  Rate, 
per  100,000  People 
per  Annum. 

f 

Hague 

From  sand  dunes 

4-7 

Pure 

Munich 

Mountain  springs 

6.0 

water  1 

Dresden 

Ground-water 

6.0 

I 

Berlin 

Filtered  water 

7.0 

Polluted  t 
water  1 

Washington 
Louisville 
Pittsburgh 

Potomac  R.  and  wells 
Ohio  River 
Allegheny  River 

71.0 
74-o 
84.0 

It  is  also  probable  that  there  is  a  relationship  be- 
tween the  annual  typhoid-fever  death  rate  in  a  city 
and  the  kind  and  amount  of  pollution  of  its  water-sup- 
ply. This  is  indicated  in  Table  II,  the  data  for  which 
have  been  compiled  from  the  records  of  a  great  many 
cities.  While  the  figures  are,  of  course,  approximate, 
there  is  sufficient  reasonableness  in  the  averages  to 
entitle  them  to  consideration. 


TABLE   II. 


Kind  of  Water  Used. 


Average  Typhoid-fever 

Death  Rate  per  100,000 

People  per  Annum. 


Pure  mountain  springs 6 

Properly  filtered  water 12 

Pure  ground-water 18 

Protected  impounded  supplies 25 

Large  normal  rivers 28 

Large  lakes 39 

Upland  streams 44 

Polluted  supplies 70-300+ 

Basing  calculations  on  the  above  averages,  it  will 
be  seen  that,  considering  the  water  furnished  by 
mountain  springs  as  the  purest  obtainable  for  a  city's 


INTRODUCTORY.  3 

supply,  and  expressing  the  average  annual  typhoid- 
fever  death  rate  of  a  city  using  such  water  by  i  per 
100,000  living,  the  average  rate  in  cities  using  other 
kinds  of  water  would  be  multiples  of  this  in  about  the 
following  ratios: 

TABLE   III. 

Kind  of  Water  Used  by  the  City. 

Pure  mountain  springs i 

Properly  filtered  water 2 

Pure  ground  water 3 

.Protected  impounded  supplies 4 

Large  normal  rivers 5 

Large  lakes 6 

Upland  streams 7 

Polluted  supplies   10-30 

Thus,  for  instance,  the  typhoid-fever  death  rate  in 
a  city  supplied  with  water  from  upland  streams,  with- 
out large  storage  reservoirs,  would  be  expected  to  be 
about  seven  times  as  great  as  if  the  water-supply  were 
from  pure  mountain  springs;  and  the 'filtration  of  such 
water  would,  on  the  average,  prevent  about  three 
fourths  of  the  typhoid-fever  deaths. 

THE  PURIFICATION  OF  WATER  BY  NATURAL  AGENCIES. 

In  polluted  waters  a  considerable  amount  of 
purification  may  take  place  from  natural  causes. 
Among  these  are  sedimentation,  chemical  changes, 
the  action  of  certain  vegetal  growths  in  promoting 
sterilization,*  and  the  action  of  certain  bacteria  to- 

*  See  page  188. 


4  WATER  FILTRATION    WORKS. 

ward  liquefying  and  nitrifying  the  organic  matter 
present  in  the  water. 

Aeration. — The  aeration  of  water,  by  passing  it  over 
cascades,  or  falls,  is  popularly  supposed  to  do  much 
toward  its  purification.  The  greatest  fields  of  useful- 
ness for  this  treatment,  however,  are  for  the  oxidation 
of  iron  in  solution;  the  removal  of  disagreeable  gases; 
the  prevention  of  stagnation,  and  the  retardation  of 
the  growth  of  certain  forms  of  vegetal  life  in  the  wa- 
ter, which,  by  their  development,  impart  disagreeable 
odors  and  tastes. 

The  waters  from  the  deep  wells  in  New  Jersey  fre- 
quently contain  iron  in  sufficiently  large  quantities  to 
give  them  a  disagreeable  taste  and  to  render  them 
unfit  for  use  for  many  purposes.  These  troubles  may 
often  be  removed  by  simple  aeration  accompanied  by 
a  rapid  filtering  process  to  remove  the  iron  salts. 
Quite  extensive  plants  of  this  kind  are  in  operation  at 
Atlantic  Highlands  and  Asbury  Park.  If  the  iron  is 
present  in  the  form  of  sulphates,  however,  simple 
aeration  is  not  so  effective,  and  a  treatment  of  the  wa- 
ter by  the  addition  of  milk  of  lime,  followed  by  aera- 
tion 'and  rapid  filtration,  proves  successful.  This 
treatment  was  resorted  to  at  Reading,  Massachusetts. 

At  Koenigsberg,  Germany,  the  water  supplied  to 
the  city  is  allowed  to  flow  about  five  miles  in  a  natu- 
ral watercourse  to  effect  the  removal  of  the  iron. 
The  iron  is  deposited  on  the  bottom  and  the  water 
issues,  clear  and  bright,  at  the  lower  end  of  the  open 
channel. 

Generally  speaking  aeration  is  ineffective  except 


ItfTKODUCTORY.  S 

for  the  purposes  stated;  sometimes  it  may  even  have 
the  opposite  effect  to  purification.  For  instance:  In 
1897  Professor  Albert  R.  Leeds  found  that  aeration 
of  the  Brooklyn  water  favored. the  growth  of  Aste- 
rionella,  an  organism  that  has  caused  much  trouble, 
at  certain  seasons,  by  imparting  a  very  disagreeable 
taste  and  odor  to  the  water.  In  this  case  it  was  found 
that  the  multiplication  of  Asterionella  was  favored, 
essentially,  by  abundant  access  of  light;  by  a  gentle 
tremulous  motion  of  the  water;  by  the  absence  of 
peaty  or  other  coloring  matter  in  the  water,  and  by 
storage  in  shallow  reservoirs,  together  with  the  pres- 
ence of  silica  and  nitrogenous  food  matter.  So  far  as 
was  known,  the  only  remedy  which  proved  effectual 
was  the  exclusion  of  light.  In  order  to  avoid  this 
trouble,  in  the  case  of  the  Brooklyn  water,  a  by-pass 
was  provided,  so  that  the  reservoirs  in  which  Aste- 
rionella caused  most  trouble  could  be  cut  out  of  the 
distribution  system  temporarily  if  necessary. 

Effects  of  Storage. — Ground- waters  and  filtered  wa- 
ters should  generally  be  stored  in  dark  reservoirs,  and 
should 'be  delivered  to  the  consumers  as  quickly  as 
possible,  as  they  nearly  always  deteriorate  during 
storage  and  upon  exposure  to  light.  Surface  waters, 
however,  are  frequently  improved  in  quality  by  stor- 
age in  large,  deep  reservoirs,  particularly  if  the  reser- 
voir sites  have  been  cleared  of  vegetation  and  top 
soil  before  the  reservoirs  are  filled,  and  if  the  feeding 
streams  are  somewhat  turbid.  Under  these  condi- 
tions the  polluting  matter  washed  into  the  reservoirs 
is,  greatly  dispersed;  from  75%  to  90%  of  the  sus- 


o  WATER  FILTRATION  WORKS. 

pended  matter,  together  with,  often,  as  much  as 
80%  to  90%  of  the  microscopic  vegetal  and  animal 
organisms,  settles  to  the  bottom  and  the  water  is  left 
nearly  free  from  turbidity  and  objectionable  qualities. 
The  absence  of  decomposing  organic  matter  in  the 
bottom  of  the  reservoir,  if  stripped,  deprives  the 
water  of  the  nitrogenous  and  carbon  compounds  nec- 
essary to  support  the  life  of  these  microscopic  organ- 
isms, and,  hence,  they  will  not  multiply  rapidly. 
Light  is  necessary  to  promote  the  growth  of  most 
organisms,  but  to  certain  forms  it  is  fatal.  Janowski 
demonstrated  that  gelatine  freshly  inoculated  with 
typhoid  germs  developed  colonies  in  the  dark  in  three 
days,  in  diffused  daylight  in  five  days,  but  that  in 
strong  sunlight  the  gelatine  became  sterile  in  six 
hours. 

Effects  of  Mud  Deposits. — Deposits  of  mud  in  stor- 
age reservoirs  are  not  necessarily  harmful.  In  Phila- 
delphia, when  the  Lehigh  basin  was  emptied  in  1886, 
an  analysis  of  the  water  covering  the  mud  showed  no 
injurious  constituents.  The  same  results  were  ob- 
tained at  the  Fairmount  reservoir,  when  emptied  re- 
cently to  permit  the  making  of  repairs,  and  also  at  the 
Mt.  Airy  basin,  which  had  not  been  cleaned  for  thirty 
years,  and  contained  from  four  to  five  feet  of  mud. 

Plate  I  shows  this  reservoir  when  the  mud  had  been 
partially  removed. 

Such  deposits  in  shallow  reservoirs  may,  however, 
by  furnishing  proper  food,  encourage  growths  of  or- 
ganisms that  will  by  their  development  impart  dis- 
agreeable tastes  and  odors.  Mr.  George  C.  Whipple, 


INTRODUCTORY.  g 

Director  of  the  Mt.  Prospect  Laboratory  of  the 
Brooklyn  Water-supply,  and  D.  D.  Jackson,  chemist, 
have  found  that  such  was  the  case  in  some  of  the 
Brooklyn  Reservoirs,*  and  have  recommended,  as  a 
remedy,  the  cleaning  of  the  reservoirs  early  in  the 
spring  and  late  in  the  summer. 

Effects  of  the  Fouling  of  Water-mains. — The  fouling 
of  the  water-mains  in  Philadelphia,  by  deposits  in 
them  and  by  growths  on  their  interior  surfaces,  was 
considered  by  some  people  the  cause  of  the  poor 
quality  of  the  water;  and  considerable  pressure  was 
brought  to  bear  to  have  the  mains  cleaned,  in  order  to 
increase  the  quantity  of  water  they  would  deliver 
and  also  to  improve  its  quality.  The  available  data 
concerning  the  effects  of  the  fouling  of  water- 
mains  rather  strongly  indicate  that  the  quality  of 
the  water  improves  in  its  passage  through  the  dis- 
tribution pipes.  Mr.  George  C.  Whipple  found  that 
during  the  passage  of  surface  waters  through  pipe 
lines  in  Boston  there  was  a  considerable  reduction  in 
the  number  of  organisms,  due  to  sedimentation,  dis- 
integration, decomposition  and  consumption  by  other 
organisms;  that  there  was  also  a  similar  decrease  in 
the  number  of  bacteria,  except  during  periods  of  the 
year  when  decomposition  was  going  on  in  the  pipes, 
and  that  by  their  decay  the  growths  tended  to  pro- 
duce bad  odors  and  tastes.  These  rank  growths  are 


*  Asterionella;  its  Biology,  its  Chemistry,  and  its  Effects  on 
Water-supplies.  Geo.  C.  Whipple  and  D.  D.  Jackson,  Journal  of 
the  New  England  Water-works,  Assn.,  Vol.  XIV,  No.  I. 


lo  WATER  FILTRATION 

not  found  in  pipes  carrying  filtered  water,  or  ground- 
water  free  from  microscopic  forms,  as  such  waters  do 
not  furnish  the  necessary  food-supply. 

Incrustation  is  generally  greatest  with  clear,  bright 
waters  because  they  contain  much  oxygen  and  car- 
bonic acid  which,  in  the  absence  of  mineral  matter,  are 
left  free  to  attack  the  pipes.  When  pipes  become  se- 
riously incrusted,  so  that  their  capacity  is  reduced, 
they  may  either  be  cleaned  or  duplicated.  The  clean- 
ing of  large  mains  is  done  with  a  scraper,  a  steel  tool, 
which  is  inserted  in  the  pipe  and  forced  through  by  the 
pressure  of  the  water.  Pipes  smaller  than  five  inches  in 
diameter  cannot  be  readily  cleaned  in  this  way  unless 
the  pressure  is  very  high.  The  cleaning  is  generally 
done  in  the  night-time,  when  there  is  little  noise  from 
traffic,  as  the  movement  of  the  scraper  in  the  main 
must  be  located  by  the  noise  it  makes  in  remov- 
ing the  blisters. 

The  scraping  of  small  pipes,  by  hand,  is  said  to  cost 
on  the  average  about  one  cent  per  foot  per  inch  of 
diameter. 

The  cost  of  cleaning  several  large  water-mains,  in- 
cluding labor,  the  cost  of  putting  in  special  manholes 
for  entering  the  scraper,  etc.,  is  given  in  Table  IV. 

The  danger  of  using  lead  pipe  with  very  soft  water 
is  well  recognized.  In  such  waters  there  is  often 
enough  free  acid  to  attack  the  lead  and  thus  form 
poisonous  salts.  The  following  case  was  reported  in  a 
recent  number  of  the  Gesundheits-Ingenieur.  Water 
was  brought  in  a  lead  pipe  to  a  forester's  lodge,  at  a 
certain  bathing  resort  in  Germany,  from  a  spring 


INTRODUCTORY. 


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12  WATER  FILTRATION    WORKS. 

about  two  hundred  feet  distant.  On  investigation  it 
was  shown  that  the  plumber,  when  soldering  the 
joints,  had  allowed  a  lot  of  lead  filings  to  remain  in 
the  pipe.  The  daughter  of  the  forester  became  ill 
some  days  after  the  completion  of  the  work,  the  ill- 
ness proving  fatal  shortly  afterward.  A  post-mortem 
examination  demonstrated  the  presence  of  lead  in  sev- 
eral of  the  organs  of  her  body,  and  the  water,  which 
was  very  pure  at  the  spring,  was  found  to  contain 
0.95  mg.  of  dissolved  lead  per  litre. 

Self -purification  of  Streams. — It  is  popularly  be- 
lieved that  running  water,  after  a  few  miles  of  flow, 
will  purify  itself  to  a  high  degree.  As  a  matter  of  fact 
the  amount  of  purification  that  takes  place,  naturally, 
in  a  running  stream  is  quite  limited,  so  far  as  the  dis- 
appearance of  the  disease  germs  is  concerned.  Their 
dispersion  through  the  mass  of  the  water,  and  the 
greater  dilution  caused  by  the  increasing  volume  of 
flow,  are  probably  the  greatest  factors  in  the  apparent 
lessening  of  pollution  toward  the  mouths  of  rivers. 

Some  of  the  influences  tending  toward  self-purifica- 
tion are  referred  to  on  page  188. 

Effects  of  Freezing. — Freezing  will  not  purify  or 
render  safe  for  use  water  which  has  previously  been 
polluted  with  fecal  matter.  Ordinarily  it  has  been  ob- 
served that  clear,  transparent  ice,  when  melted,  yields 
about  ten  per  cent,  as  many  bacteria  as  were  present 
in  the  raw  water;  or,  in  other  words,  the  percentage  of 
removal  of  the  organisms  would  be  comparable  to 
that  obtained  by  sedimentation  in  large  reservoirs. 


IN  TROD  UCTOR  Y.  13 


THE  PROTECTION  OF  WATER-SUPPLIES. 

Permissible  Pollution. — There  are  certain  conditions 
under  which  the  pollution  of  a  stream  is  permissible. 
This  right  is  recognized  in  various  States  by  the  Mill 
Acts,  which  are  intended  to  foster  the  development  of 
industries.  These  acts  could  not  be  operative  unless 
the  right  of  stream-pollution  were  conceded,  to  a  cer- 
tain extent.  The  right  to  prevent  the  discharge  of 
sewage  into  a  river,  when  it  would  result  in  a  public 
nuisance,  is  now  well  established.  The  Supreme 
Court  of  Connecticut  has  recently  restrained  the  cities 
of  Danbury,  Waterbury,  and  New  Britain  from  dis- 
charging their  sewage  into  the  Still  River,  Naugatuck 
River,  and  Pipers  Brook,  respectively,  because  of  the 
creation  of  nuisances.  Similar  decisions  have  been 
made  in  other  States.  In  many  cases  these  decisions 
prepare  the  way  for  the  collection  of  damages,  the 
cities,  or  parties  causing  the  nuisances,  finding  it 
sometimes  more  convenient  to  pay  the  damages-  an- 
nually than  to  put  in  the  costly  works  necessary  to 
correct  the  evils. 

Legal  Protection  of  Water-supplies. — The  law  under 
which  New  York  City  maintains  the  purity  of  its  wa- ' 
ter-supply  gives  the  city  power  to  obtain  title  to  all 
lands  in  the  Croton  watershed  necessary  for  the 
construction  of  dams,  reservoirs  and  appurtenances, 
and  also  the  power  to  secure  title  to  strips  of  land 
around  the  edges  of  these  reservoirs,  and  the  banks  of 
the  streams  feeding  them,  to  insure  the  sanitary  pro- 


14  WATER   FILTRATION    WORKS. 

tection  of  the  water.  A  further  act,  passed  in  1893, 
gives  the  city  power  to  acquire  the.  title  to  any  real 
estate  "  for  the  sanitary  protection  of  all  rivers  and 
other  watercourses,  lakes,  ponds  and  reservoirs  in 
the  counties  of  Westchester,  Dutchess  and  Putnam, 
so  far  as  the  same  now  are,  or  hereafter  may  be,  used 
for  the  supply  of  water  for  the  City  of  New  York." 
Unfortunately,  as  the  laws  now  stand,  New  York  City 
is  unable  to  go  to  other  water  sources  for  an  addi- 
tional supply,  but  active  measures  are  being  taken  to 
remedy  this  fault  so  that  this  great  Metropolitan  Dis- 
trict may  have  powers  similar  to  those  granted  to 
other  cities  of  the  State. 

In  1898  a  law  was  passed  which  gives  any  town  in 
New  York  State  the  privilege  of  purchasing  the  prop- 
erty and  franchise  of  its  water-works,  at  any  time  the 
company  may  be  willing  to  sell,  at  a  price  to  be 
agreed  upon,  the  city  being  empowered  to  bond 
itself  to  pay  for  the  works,  and  also  to  assume  their 
indebtedness  and  to  operate  them. 

Provisions  for  Betterment  of  Water-supplies. — While 
all  these  laws  regarding  the  protection  of  water-sup- 
plies have  been  drawn  up  with  a  view  of  throwing  safe- 
guards around  the  public  health,  the  time  has  come 
when  we  are  forced  to  realize  that  legal  enactments  of 
this  kind  are  inadequate,  and  that  the  way  must  be 
prepared  to  permit  the  citizens  of  any  city  or  town  fo 
secure  water  as  pure  as  it  is  possible  to  make  it.  It  is 
also  true  that  legislation  tending  toward  that  end 
must  be  secured  through  the  exercise  of  great  discre- 
tion and  wisdom;  any  actions  that  would  disturb  con- 


INTRODUCTORY.  1 5 

fidence  in  investments,  affect  the  market  values  of 
stocks  and  bonds,  or  raise  questions  as  to  the  ability 
of  concerns  already  in  operation  to  pay  fixed  charges 
and  customary  dividends,  would  be  met  with  deter- 
mined opposition. 

Requirements  of  Water  Companies  in  the  Matter  of 
Protection. — The  laws  of  to-day  require  a  water  com- 
pany to  exercise  only  ordinary  and  reasonable  care  in 
the  protection  of  its  water;  and  it  has  been,  thus  far, 
impossible  to  fasten  upon  a  company  or  corporation 
the  responsibility  for  the  deaths  of  persons  resulting 
from  the  drinking  of  such  water.  It  is  not  difficult  to 
see  why  this  should  be  so.  In  order  that  the  company 
might  be  forced  to  pay  damages  it  would  have  to  be 
shown,  among  other  things,  that  there  was  an  in- 
tent to  defraud  or  deceive;  that  there  had  been 
criminal  negligence  in  the  care  of  the  supply;  that 
the  water  was  unquestionably  the  cause  of  the  ill- 
ness or  death  in  question,  and  that  there  had  not 
been  contributory  negligence  on  the  part  of  the 
user.  When  the  pollution  of  the  water  is  a  mat- 
ter of  common  knowledge,  fully  discussed  in  pub- 
lic and  in  the  press,  and  when  the  deceased  had 
knowledge  of  the  pollution,  it  has  been  held  that 
drinking  of  the  water  was  contributory  negligence. 
If  the  pollution  were  of  an  accidental  nature  the  diffi- 
culty would  lie  in  proving  that  the  water  was  the 
cause  of  the  trouble  in  that  specific  case,  as  there  are 
many  well-known  agents,  other  than  water,  by  means 
of  which  disease  may  be  spread. 

Protection  of  Surface  Supplies. — Generally  speaking. 


1 6  WATER  FILTRATION    WORKS. 

only  those  supplies  derived  from  surface  gathering 
grounds,  small  rivers,  and,  to  a  certain  extent,  those 
from  ground-water,  can  be  protected  by  legislative  or 
administrative  action.  As  regards  protection,  large 
rivers  are  in  a  different  class  from  small  streams. 
Some  sanitarians  hold  that  the  upland  streams  should 
be  regarded  as  the  water-supply  sources  of  a  land,  and 
the  large  streams  as  its  sewers,  recommending  legis- 
lative action  to  govern  the  protection  of  the  small 
streams  and  to  determine  the  degree  of  permissible 
pollution  of  the  large  ones.  There  are  many  difficul- 
ties in  the  way  of  securing  uniform  legislation  on  this 
question,  because  of  the  great  diversity  of  interests 
affected,  and  the  difficulty  of  drawing  the  line  be- 
tween large  and  small  rivers.  The  question  must  be 
specially  solved  for  each  locality  by  the  adoption  of 
that  plan  which  will  be  of  most  benefit  to  all  interests. 
On  a  very  large  river,  for  instance,  it  would  be  far 
cheaper,  and  would  afford  greater  security  to  the  peo- 
ple, for  all  the  cities  to  discharge  their  sewage  into 
the  river  unpurified,  and  then  purify  the  drinking- 
water  drawn  from  it,  than  for  them  to  purify  their 
sewage  and  then  drink  the  untreated  river  water. 
Under  some  conditions  it  might  be  necessary  to 
purify  both  the  sewage  and  drinking-water. 

The  topographical  conditions  on  small  highland 
streams  are  not  generally  favorable  for  the  develop- 
ment of  large  industries,  and  it  is  possible,  therefore, 
on  such,  to  enforce  restrictions  against  pollution  be- 
cause on  the  small  streams  the  conflicts  of  interests 
are  not  so  great  as  on  the  large  lowland  rivers. 


INTRODUCTORY.  ';     .  I/ 

Ownership  vs.  Legal  Protection. — Absolute  owner- 
ship of  the  watershed  is,  by  some,  considered  more 
effective  than  its  protection  by  legal  enactments. 
Manchester  and  Liverpool  own  the  watersheds  from 
which  their  water  is  derived,  and  recently  Mr.  James 
Mansergh,  President  of  the  Institution  of  Civil  En- 
gineers, has  recommended  to  Birmingham  the  pur- 
chase of  the  45,000  acres  of  land  from  which  its  water- 
supply  is  drawn.  Edinburgh  and  Glasgow,  however, 
protect  their  watersheds  by  legal  enactments  and  by 
contracts  with  the  landowners.  In  this  country  the 
water  companies  generally  own  only  the  land  upon 
which  the  reservoirs  and  buildings  are  situated,  ob- 
taining easements  for  right  of  way  for  pipe  lines,  etc., 
and  in  some  of  the  States  depend  upon  the  legal  pow- 
ers conferred  upon  the  Board  of  Health  to  prevent 
the  pollution  of  the  water.  In  some  cases  these  pow- 
ers are  quite  ample,  while  in  others  they  are  practi- 
cally inoperative.  It  is  not  yet  clearly  established 
that  ownership  of  the  watershed  permits  of  greater 
protection  of  the  water-supply  than  can  be  obtained 
by  legal  enactments  without  such  ownership.  Judg- 
ing by  their  typhoid-fever  death  rates,  Manchester 
and  Liverpool,  owning  their  watersheds,  do  not  seem 
to  be  more  effectively  protected  against  typhoid 
fever  than  Brooklyn,  New  York,  Glasgow  or  Boston, 
having  control  over  their  supplies  mainly  by  legal 
powers. 

Effects  of  Surface  Washings. — It  is  becoming  pretty 
well  recognized  that  surface  washings  are  an  import- 
ant factor  in  the  pollution  of  water-supplies.  It  is 


i8 


WATER  FILTRATION   WORKS. 


quite  interesting,  in  this  connection,  to  note  that  the 
annual  typhoid-fever  death  rates  of  New  York,  Bos- 
ton, Cleveland,  Detroit,  Columbus,  Louisville,  Pater- 
son,  Pittsburgh,  San  Francisco,  and  Toledo  have,  at 
various  times,  fluctuated  synchronously  for  a  num- 
ber of  years  with  the  annual  rainfall  for  the  corre- 
sponding years;  indicating  that  the  typhoid  fever  in 
these  cases  was  proportional  to  the  amount  of  pollut- 
ing matter  washed  into  their  respective  sources  of 
supply. 

The  available  evidence  goes  to  show  that  legal  pro- 
tection of  a  water-supply  may  effect  a  considerable  re- 
duction in  the  death  rate  of  a  city,  but  that  such  pro- 
tection cannot  guarantee  a  water  as  pure  as  spring- 
water  or  properly  filtered  water.  The  logical  de- 
duction is,  therefore,  that  in  most  cases  filtration  of 
the  water  will  be  required  where  there  is  danger  of 
sewage-pollution.  Of  course,  as  a  matter  of  precau- 
tion, all  safeguards  should  be  put  in  force  in  such  mat- 
ters, and  the  direct  sewage-pollution  of  a  body  of  wa- 
ter, intended  for  use  as  a  source  of  supply,  should,  if 
possible,  always  be  prohibited,  whether  or  not  filtra- 
tion is  subsequently  employed.  It  is  also  well  estab- 
lished that  the  purification  of  the  sewage  of  cities,  be- 
fore discharging  it  into  a  stream  subsequently  used  as 
a  source  of  supply,  will  be  less  effective  as  a  health 
preservative  measure  and  less  feasible  from  a  financial 
point  of  view  than  the  purification  of  the  drinking-wa- 
ter supplies  drawn  therefrom.  Undoubtedly  there 
will  be  many  situations  where  both  processes  will  be 
necessary. 


INTROD  UCTOR  Y.  1$ 

It  has  been  the  author's  firm  conviction  for  some 
years  that  the  time  is  not  far  distant  when  the  public 
will  demand  the  purification  of  all  supplies  derived 
from  surface  gathering-grounds,  when  practicable. 
A  similar  view  was  expressed  in  the  report  of  the 
Mayor's  Expert  Water  Commission,*  of  Philadelphia, 
in  reporting  upon  the  Filtration  of  the  Philadelphia 
Supply. 

The  available  statistics  indicate  that  surface-water 
supplies,  except  those  which  have  enormous  storage 
reservoirs,  cannot  be  generally  regarded  as  safe. 
Large  reservoirs  afford  considerable  protection  to 
surface-water  supplies.  The  great  capacity  of  the 
reservoirs  permits  the  impurities  washed  into  them 
to  be  thoroughly  dispersed,  favors  the  sedimentation 
of  a  large  percentage  of  the  bacteria,  and  furnishes 
favorable  conditions  for  the  oxidation  and  absorp- 
tion of  the  nitrogenous  matter  in  the  water  by 
aquatic  plants  and  microscopic  life. 

Large  surface  supplies  are  generally  more  difficult 
to  protect  than  small  ones.  The  actual  protective 
measures  are  naturally  divided  into  two  classes:  those 
which  must  be  used  when  the  works  are  building, 
and  those  which  must  be  enforced  after  the  works  are 
placed  in  operation.  When  a  supply  of  surface-water 
is  to  be  furnished,  it  is  first  necessary  to  acquire  the 
land  upon  which  the  reservoirs  will  be  situated.  In 
large  works  this  will  require  the  acquisition  of  many 

*  Report  on  the  Extension  and  Improvement  of  the  Water- 
supply  of  the  City  of  Philadelphia,  by  Rudolph  Hering,  Joseph  M. 
Wilson,  Samuel  M.  Gray. 


2O  WATER  FILTRATION   WORKS. 

farms,  and  perhaps  villages  and  towns,  and  the  de- 
struction of  many  industries.  The  land  taken  should 
include  everything  lower  than  high-water  line  of  the 
proposed  reservoir,  with  an  allowance  of  two  or  three 
feet  for  exigencies;  in  addition,  a  strip,  200  or  300 
feet  wide,  should  be  secured  all  around  the  water,  to 
give  perfect  control  over  the  shores.  The  same  policy 
should  be  followed,  if  possible,  in  regard  to  the  prin- 
cipal feeders  of  the  reservoirs.  This  property  is  se- 
cured usually  by  appraisal  by  commissioners,  and  in 
many  cases  it  results  in  a  direct  benefit  to  the  inhabi- 
tants of  the  territory  taken. 

After  the  site  has  been  acquired,  the  fences  and 
buildings  must  be  removed  to  other  sites,  or  burned, 
and  the  vegetation  must  be  cut  down  and  burned,  or 
removed.  The  necessity  of  removing  the  top  soil  and 
small  vegetation  has  in  late  years  been  given  much 
prominence.  Some  of  the  older  Boston  reservoirs 
were  not  so  treated,  and  the  great  deterioration  of  the 
water  due  to  the  slowly  decomposing  organic  matter 
was  a  source  of  much  anxiety.  This  trouble  has  been 
remedied  at  great  expense  by  drawing  down  the  wa- 
ter, pulling  out  the  stumps,  removing  the  soil,  in  some 
cases  to  great  depths,  and  paving  the  slopes.  Late 
investigations  have  shown  that  the  removal  of  from 
6  to  12  inches  of  the  top  soil  will  accomplish  all  that 
can  be  desired,  and  the  covering  of  the  mucky  places 
with  a  foot  of  gravel  has  served  as  well  as  remov- 
ing the  entire  deposit.  It  is  also  necessary  that 
the  bottom  of  the  reservoir  should  be  graded  so 
that  all  the  water  will  drain  to  the  outlet,  and  not 


INTRODUCTORY.  21 

leave  stagnant,  isolated  pools  when  the  water  is 
drawn  down.  In*  addition  to  the  treatment  of  the 
reservoir  site,  it  is  necessary  to  drain,  or  cut  off, 
the  swampy  areas  on  the  watershed  by  ditches  or 
banks.  Sometimes  a  few  ditches  satisfy  the  condi- 
tions, and  sometimes  it  is  necessary  to  convey 
the  water  feeding  the  swamp  in  direct  channels  to  a 
near-by  watercourse,  and  isolate  the  swamp  by  em- 
bankments. While  all  these  operations  are  going  on, 
and  while  the  dams  and  accessory  works  are  build- 
ing, tight  portable  earth-closets  must  be  provided 
for  the  use  of  the  workmen  and  every  precaution 
must  be  taken  to  insist  upon  their  proper  use  and  care, 
there  being  several  cases  on  record  where  epidem- 
ics have  resulted  from  the  neglect  of  this  simple  pre- 
caution. 

During  the  operation  of  the  works  the  principal 
sources  of  pollution  that  must  be  controlled  are  the 
washings  from  the  streets  and  roads;  the  washings 
from  the  fields;  the  pollution  from  swamps  and  bogs; 
the  refuse  from  manufacturing  establishments;  sew- 
age matter;  garbage;  farm  refuse,  and  the  drainage 
from  cemeteries. 

Much  of  the  pollution  from  the  street-washings,  in 
the  villages,  can  be  abated  by  efficient  street-cleaning 
methods  and  sanitary  regulations  regarding  the  col- 
lection and  disposal  of  the  refuse.  Street-sweepings 
have  a  value  and  can  readily  be  disposed  of  to  farnir 
ers,  in  the  neighborhood,  for  fertilizing  purposes. 
The  washings  from  rural  roads  can  nearly  always  be 
purified,  to  a  certain  extent,  by  diverting  the  ditch- 


22  WATER  FILTRATION   WORKS. 

water  at  intervals  over  lands  adjoining  the  road, 
whereby,  through  sedimentation,  -straining  and  par- 
tial filtration,  a  large  amount  of  the  objectionable  im- 
purities may  be  removed.  The  washings  from  culti- 
vated lands,  when  the  fertilizer  is  of  an  objectionable 
character,  should  be  spread  out  over  grass  land,  or 
passed  through  a  porous  soil,  for  a  considerable  dis- 
tance, before  being  allowed  to  flow  into  the  feeders 
of  the  supply. 

Protection  from  Sewage-pollution. — The  problem  of 
dealing  with  the  sewage  of  the  villages  resolves  itself 
generally  either  into  a  system  of  dry  removal,  or  into 
a  system  of  water  carriage,  followed  by  purification, 
before  allowing  it  to  flow  into  the  feeders  of  the  sup- 
ply. 

The  most  convenient  method  for  dry  removal  is  to 
provide  the  closets  with  coverable  pails  or  boxes,  into 
which  dry  earth  or  ashes  may  be  thrown,  as  an  ab- 
sorbent. This  method  has  been  in  use  at  Hemlock 
Lake,  N.  Y.,  the  source  of  the  domestic  supply  of 
Rochester,  since  1885,  and  has  proven  satisfactory. 
It  has  the.  ad  vantage  that  the  matter  deposited  in  the 
receptacles  can  be  kept  from  direct  contact  with  the 
air,  and  hence,  also,  away  from  flies.  Pail  contents, 
garbage,  and  other  decomposable  matter  should  be 
buried  in  a  safe  place,  or  burned  in  refuse  destructors. 
Human  faeces  should  not  be  exposed  on  the  surface  of 
the  ground  near  a  water-supply  source  nor  used  for 
fertilizing  the  soil. 

The  principal  disadvantages  of  the  pail  system  when 
applied  to  cities  are  the  great  cost  and  inconvenience 


INTRODUCTORY.  2$ 

of  the  method  as  compared  with  the  water-carriage 
system  followed  by  £he  purification  of  the  sewage. 

The  State  Boards  of  Health  generally  enact  laws  to 
prevent  the  discharge  of  sewage  from  cities  into  the 
water-supply  sources  of  other  cities,  and  therefore, 
upon  proper  complaint,  such  nuisances  may  be 
abated.  Boston,  as  is  well  known,  encourages  the 
towns  and  cities  within  the  shed  of  her  water-supply 
to  take  their  sewage  outside  the  limits,  if  possible,  or 
to  put  in  satisfactory  plants  for  purification  of  the 
same,  paying  fifty  per  cent,  of  the  cost  of  the  work, 
the  towns  paying  the  other  fifty  per  cent.  The  great- 
est difficulty  is  generally  in  enforcing  the  law,  as  this 
can  only  be  done  by  proper  legal  processes,  entailing 
often  considerable  delays. 

Protection  of  Lake  Supplies. — The  protection  of  sup- 
plies derived  from  large  lakes  is  hampered  by  many 
difficulties,  and  the  protective  measures  must  depend 
upon  the  direction  of  the  surface  and  submerged  cur- 
rents, the  size  and  growth  of  the  city,  the  relative  lo- 
cations of  the  outlets  of  sewers,  drains  and  large  pol- 
luted streams,  the  amount  .and  direction  of  the  lake 
traffic,  the  depth  of  the  water,  annual  rainfall,  and 
many  other  factors.  An  interesting  case  of  the  pollu- 
tion of  a  lake  supply  has  been  reported  by  Professor 
Gardiner  S.  Williams,  formerly  Engineer  of  the  Board 
of  Water  Commissioners  of  Detroit,  Mich.  The  sew- 
age of  Port  Huron  is  discharged  into  the  Black  River, 
a  sluggish  stream  emptying  into  Lake  St.  Clair,  60 
miles  above  Detroit.  In  1891  the  Government  com- 
menced dredging  operations  in  the  Black  River  to 


24  WATER  FILTRATION   WORKS. 

improve  navigation,  and  the  mud  taken  from  the  bot- 
tom was  dumped  into  the  St.  Clair  River,  about  60 
miles  above  the  intake  of  the  Detroit  water-works, 
which  are  just  below  Lake  St.  Clair.  Fifty  days  after 
the  dumping  of  the  first  scow-load  of  polluting  mate- 
rial from  the  Port  Huron  sewers  into  the  St.  Clair 
River,  there  were  four  deaths  from  typhoid  fever  in 
Detroit.  This  would  allow  ten  days  for  the  water  to 
flow  from  Port  Huron  to  the  intakes,  fourteen  days 
for  the  disease  to  incubate,  and  about  twenty-six  days 
for  the  disease  to  run  its  course.  Many  cases  followed 
these  four,  the  disease  disappearing  some  weeks  after 
dredging  operations  were  suspended.  The  next  year 
typhoid  appeared  again  after  the  dredging  had  begun; 
and  again  it  disappeared  when  dredging  was  stopped. 

These  conditions  have  followed  one  another  since 
that  time,  and  investigations  have  shown  that  they 
prevailed  in  previous  years.  As  far  back  as  1886  it 
was  found  that  typhoid  fever  appeared  in  Detroit  dur- 
ing those  years  when  dredging  operations  disturbed 
the  bottom  of  the  St.  Clair  River  or  Lake  St.  Clair, 
above  the  intake  of  the  water-works. 

All  lake  cities,  as  they  grow  to  larger  proportions, 
find  it  necessary  to  gradually  extend  their  intakes 
further  from  the  shores,  unless  the  purification  of  the 
water  is  determined  upon.  At  the  city  of  Chicago  the 
intake  has  been  pushed  out  successively  from  700  feet 
to  two  miles  and  then  to  four  miles.  Cleveland  has 
now  under  construction  a  new  intake  tunnel  which 
will  be  26,000  feet  long,  when  completed;  and  the  in- 
take for  the  city  of  Buffalo  has  been  extended  from 


IN  TROD  UCTOR  Y.  2$ 

330  feet  to  1,020  feet.  In  Zurich,  Switzerland,  instead 
of  extending  the  intake  further  from  the  shore  to  get 
pure  water,  a  large  plant  has  been  constructed  for 
filtering  the  entire  supply  from  the  lake. 

THE  PURIFICATION  OF  WATER  BY  FILTRATION. 

In  the  following  pages  the  works  and  operations 
necessary  for  the  purification  of  drinking-water  for 
cities  and  towns  and  large  institutions,  by  filtration, 
will  be  described  with  some  fulness.  The  science  of 
water-purification  is  still  in  process  of  development. 
Each  new  experimental  plant  brings  to  light  new  dif- 
ficulties and  new  methods  of  overcoming  them.  Ex- 
perimental work,  such  as  that  done  at  Louisville,  Cin- 
cinnati, Pittsburg,  Providence,  and  Philadelphia,  and 
now  under  way  at  New  Orleans,  is  of  incalculable 
value,  as  it  leads  to  the  discovery  of  the  proper  treat- 
ment for  the  purification  of  waters  of  different  kinds. 
Processes  that  are  applicable  for  the  treatment  of  clear 
polluted  waters  fail  entirely  with  turbid  waters;  and 
turbid  waters  themselves  vary  so  greatly  in  regard  to 
character  and  seasonal  distribution  of  sediment  that 
each  case  requires  a  special  study.  Some  clear  wa- 
ters, also,  on  account  of  rank  algse  growths,  at  cer- 
tain seasons,  must  have  special  treatment  before  they 
can  be  filtered  successfully. 

The  filters  described  at  length  in  this  work  are 
classified  under  two  heads — slow  sand-filters  and 
rapid  sand-filters.  These  terms  must  be  used  in  the 
restricted  sense;  both  refer  to  filters  in  which  the 


26 


WATER  FILTRATION   WORKS. 


filtering  medium  is  sand.  The  slow  sand-filters  may 
be,  though  they  generally  are  not,  operated  with  the 
aid  of  chemicals  for  producing  the  surface  film,  while 
the  rapid  sand-filters  can  only  be  efficient  by  using  a 
coagulant,  such  as  aluminum  hydrate,  to  form  the 
film  artificially  and  rapidly. 

Other  types,  such  as  the  Fischer  or  Worms  filter, 
using  slabs  of  concrete,  and  the  Pasteur-Chamber- 
land,  using  tubes  of  unglazed  porcelain,  belong  in  a 
different  class. 

There  are  also  slow  sand-filters  operated  in  con- 
nection with  a  coagulant;  such  as  the  filters  at 
Antwerp,  where  the  coagulant  is  ferric  hydrate,  pro- 
duced by  the  Anderson  process,  and  the  experimental 
filters  tested  at  Cincinnati  by  Mr.  Fuller,  and  called 
by  him  "  Modified  English  Filters."  There  is  also  the 
process  used  in  the  Maignen  system,  in  which  a  layer 
of  asbestos  forms  the  surface  film  over  the  sand. 

The  indications  are  that  the  development  of  some 
preliminary  process  for  straining  out  the  finely  di- 
vided particles  of  clay,  by  the  use  of  prepared  sponges, 
layers  of  cloth,  or  other  absorbent  materials,  instead 
of  using  a  coagulant,  may,  in  the  future,  play  an  im- 
portant part  in  water-purification. 

As  a  rule,  most  waters  which  would  be  used  for  a 
water-supply  require  the  same  cycle  of  operations  to 
render  them  fit  for  use  as  drinking-water;  that  is,  the 
removal  of  turbidity,  color,  and  pathogenic  bacteria. 
These  operations  usually  require  works  for  removing 
the  suspended  matter  by  sedimentation,  with  or  with- 
out the  coagulation  of  the  finer  particles;  the  removal 


IN  TROD  UCTOR  F.  .i  27 

of  color  and  pathogenic  bacteria  by  filters,  with  or 
without  the  aid  of  coagulation,  and  the  storage  of  the 
filtered  water  in  sufficient  quantities  to  permit  the 
filters  to  operate  at  a  nearly  uniform  speed,  although 
the  draft  on  the  works  may  vary  considerably  in  rate 
at  different  times  of  'the  day.  These  different  works 
will  therefore  be  discussed  in  the  subsequent  pages  in 
the  following  order: 

Intakes. 

Sedimentation. 

Settling  basins. 

The  purification  of  water  by  slow  sand-filtration. 

The  design,  construction  and  operation  of  slow 
sand-filters. 

The  purification  of  water  by  rapid  sand-filtration. 

The  construction  and  operation  of  rapid  sand-filters. 

Other  methods  of  filtration. 

Filtered-water  reservoirs. 


CHAPTER  II. 

INTAKES,  SEDIMENTATION,  AND   SETTLING 
BASINS. 

INTAKES. 

Flowing  waters  may  be  divided  into  two  general 
classes:  those  in  which  tidal  influences  may  cause  a 
reversal  of  current,  or  at  least  a  checking  of  velocity, 
and  those  in  which  the  flow  is  continuous  in  one  di- 
rection. 

Tidal  Streams. — Water-supplies  taken  from  streams 
subjected  to  tidal  reversals  of  current  are  usually  also 
sewage-polluted,  and,  therefore,  in  the  location  of  the 
intake  due  regard  must  be  had  to  the  time  of  collec- 
tion of  the  water  to  insure  that  it  may  be  taken  only 
when  it  is  at  its  best.  The  intake  for  the  Antwerp 
works  is  at  Waelhem,  a  small  village  about  eight  miles 
to  the  south,  on  the  banks  of  the  River  Nethe.  About 
two  miles  above  Waelhem  the  Nethe  is  joined  by  two 
streams,  the  Seine,  upon  which  Brussels  is  situated, 
and  the  Dyle,  flowing  through  Malmes.  Below  the 
junction  the  river  is  called  the  Rupel;  this  flows  into 
the  Scheldt,  upon  which  Antwerp  is  situated.  The 
range  of  tide  at  Waelhem  is  about  13  feet  6  inches.  In 
order  to  avoid  taking  in  the  polluted  waters  of  the 

28 


INTAKES.  29 

Rupel,  as  they  flow  past  Waelhem,  on  the  flood  tide, 
and  the  waters  of  the  Nethe,  contaminated  at  low 
water  with  the  sewage  of  the  towns  situated  above  the 
intake,  the  water  is  let  into  the  settling  basins  three 
hours  after  high  water.  The  bottom  of  the  intake  is 
.33  foot  above  low  tide.  Water  taken  under  these 
conditions  and  purified  by  ordinary  sand-filtration 
was  pronounced  by  the  authorities  sufficiently  good 
for  the  supply  of  the  city.  At  Shanghai  the  water  is 
taken  from  the  River  Huang  Poo,  a  branch  of  the 
great  Yang-tse-Kiang.  The  range  of  the  spring 
tides  is  from  8  to  9  feet.  The  intake  for  the  water- 
works is  located  below  the  city  of  Shanghai,  where  the 
great  dilution  from  the  Yang-tse-Kiang,  on  flood 
tide,  and  the  wide  section  of  the  river,  make  the  dan- 
ger from  pollution  less  than  if  the  intake  were  above 
the  city,  where  the  river  section  is  very  much  smaller. 
The  valves  of  the  intakes  are  placed  two  feet  above 
the  low-water  level,  so  that  no  water  can  enter  them 
until  fully  one  hour  after  flood  tide  has  set  in.  By  this 
means  the  sewage  of  the  city  and  its  suburbs  is  washed 
into  the  upper  reaches  of  the  river  during  the  time  the 
water  is  being  taken  into,  the  settling  basins.  What- 
ever sewage  matter  may  have  gone  down  the  river  on 
the  previous  tide  is  so  greatly  dispersed  in  the  waters 
of  the  Yang-tse-Kiang  that  it  is  hardly  detectable. 
,  Rivers  with  Stable  Banks  above  Flood  Height,  and 
zvith  small  Range  of  Fluctuation  of  Level. — If  a  river 
has  stable  banks,  at  a  slight  elevation  above  high 
water,  a  fair  velocity,  with  a  small  range  of  fluctuation 
of  surface  elevation,  the  intake  should  be  constructed 


30  WATER  FILTRATION    WORKS. 

with  the  bottom  low  enough  to  collect  the  water  at  all 
times,  and  should  consist  of  one  or  more  pipes  or  con- 
duits ending  in  a  chamber  at  the  face  of  the  bank. 
The  distribution  of  sediment  in  rivers,  both  in  the  ver- 
tical and  horizontal,  is  discussed  on  pages  35  and  36. 
Movable -duplicate  screens  in  the  chamber  will  pre- 
vent the  entrance  of  floating  and  other  objects  that 
might  interfere  with  the  valves  of  the  pumps.  At 
Berlin,  at  the  Mueggle  See  works,  on  account  of  the 
shallow  water  and  sloping  bottom,  the  water  near  the 
shore  is  generally  muddy  from  the  action  of  the 
waves.  To  secure  clearer  Avater  the  bottom  of  the 
lake  was  dredged  to  a  depth  of  6.6  feet  in  front  of  the 
works,  for  a  distance  of  about  400  feet  from  the  shore 
to  the  point  where  the  bottom  drops  off  rapidly  to  a 
depth  of  about  26  feet.  The  intakes  are  box  con- 
duits, with  a  sectional  area  of  about  24  square  feet 
each,  built  up  of  oak  planks,  and  extending  from  the 
deep  water  to  the  screen  wells  or  shafts  at  the  shore. 

Rivers  with  Stable  Banks  bclozv  Flood  Height. — If  the 
river  'has  stable  banks,  which  are  below  flood  height, 
and  the  works  are  protected  by  levees  and  dikes,  the 
intake  may  still  be  as  above  described,  but  it  will  then 
be  necessary  to  provide  a  gate  in  the  conduit,  placed 
in  a  manhole  located  in  the  dike,  in  order  to  regulate 
the  amount  of  water  admitted  to  the  works  when  the 
river  is  very  high,  as  was  done  at  Hamburg.  If  the 
gate  were  located  behind  the  dike  the  hydrostatic 
pressure  on  the  inside  of  the  conduit  might  cause  its 
rupture  if  it  were  of  masonry  construction. 

If  the  settling  basins  are  lower  than  the  river,  in  ad- 


PLATE  TI. — INTAKE  OF  THE  ST.   Louis,  Mo.,  WATER  WORKS. 


SED I  MEN  TA  TION.  3  3 

dition  to  the  gate  in  the  conduit  in  the  dike,  referred 
to  above,  it  may  sometimes  be  necessary  to  provide  a 
reflex  gate  or  valve  to  prevent  the  water  from  escap- 
ing to  the  river,  in  case  the  attendants  should  'neglect 
to  close  the  gate  when  the  basins  were  rilled.  If  the 
basins  are  to  be  arranged  to  be  flooded  quickly  when 
the  water  is  at  its  best,  a  relief  pipe  must  be  built  to 
provide  means  for  the  escape  of  the  air  contained  in 
the  conduit,  thus  avoiding  the  dangers  incident  to 
concussion,  as  was  done  at  Antwerp. 

Rivers  with  Shifting  Banks  and  Bottoms,  and  Great 
Fluctuation  of  Level. — When  the  river  has  shifting 
banks  and  bottom,  and  the  range  of  fluctuation  of 
surface  level  is  great,  intakes  become  very  expensive 
structures.  The  intake,  in  this  case,  must  start  from 
a  point  in  the  bed  of  the  stream  where  the  chan- 
nel is  permanent,  and  should  consist  of  a  masonry  or 
other  heavy  structure,  resting  on  a  firm  foundation, 
and  constructed  with  a  view  of  resisting  the  action  of 
the  water,  ice  and  floating  objects.  Under  these 
conditions  pumping  must  always  be  resorted  to  be- 
tween the  river  and  the  settling  basins.  The  conduit 
from  the  intake  to  the  pump-well  must  pass  under 
the  bed  of  the  river,  and  may  be  enlarged,  before 
reaching  the  pumps,  to  form  the  screening  chamber. 
The  intake  at  the  St.  Louis  water-works  is  of  this 
type. 

SEDIMENTATION. 

r Amount,  Character  and  Distribution  of  Sediment. — 
The  purification  of  polluted  water  may  require  the 


34  WATER  FILTRATION   WORKS. 

removal  therefrom  of  suspended  particles  of  finite  di- 
mensions, matters  in  solution,  and  microscopic  ob- 
jects, both  animate  and  inanimate.  For  the  removal 
of  the  first  class  of  matter,  straining,  or  sedimentation 
alone,  or  combinations  of  these  methods,  will  gener- 
ally suffice;  for  the  second  and  third  classes,  chemical 
or  mechanical  treatment,  or  some  method  of  filtra- 
tion, will  probably  be  necessary. 

Waters  taken  from  large  lowland  rivers  flowing 
through  valleys  or  plains,  formed  of  the  detritus  and 
washings  of  the  highlands,  carry,  at  all  seasons,  large 
quantities  of  matter  in  suspension.  A  certain  part 
of  this  matter  can  be  removed  by  allowing  the  water 
to  stand  in  comparative  quiescence  in  large  settling 
basins  or  reservoirs.  The  amount  of  matter  that  can 
be  carried  in  suspension  depends  on  the  viscoscity  of 
the  water;  the  chemical  composition  and  degree  of 
comminution  of  the  matter  in  suspension;  the  eddies 
caused  by  the  deflection  of  the  strata  of  water  by  im- 
pingement against  the  bottom  and  sides  of  the 
stream;  vortex  motion,  and  probably  on  other  imper- 
fectly understood  causes.  It  will  be  seen  therefore 
that  a  river  may  carry  different  amounts  of  matter  in 
suspension  at  different  periods  of  the  year,  and  at  dif- 
ferent portions  of  its  course.  This  is  well  illustrated 
in  the  case  of  the  Mississippi  River  and  its  tributaries, 
the  estimated  average  turbidity  in  parts  per  million 
of  the  Allegheny  at  Pittsburgh  being  given  as  50; 
the  Ohio  at  Cincinnati  as  230;  the  Ohio  at  Louisville 
as  350,  and  the  Mississippi  at  New  Orleans  as 
560,  The  average  estimated  turbidity  of  the  Merri- 


SEDIMENTATION.  35 

mac  at  Lawrence  is  given  as  10;  of  the  Hudson  at 
Albany  as  15  and  the  Potomac  at  Washington  as  80 
parts  per  million  respectively.*  The  quantity  of  sedi- 
ment carried  in  flowing  waters  is  discussed  more  at 
length  on  pp.  64  et  seq.  In  large  rivers,  flowing 
through  lowlands,  it  has  been  frequently  observed 
that  the  amount  of  suspended  matter  gradually  de- 
creases, per  unit  of  volume  of  water,  toward  the  em- 
bouchure, though  this  may  not  always  be  the  case. 
It  has  also  been  observed  that  the  weight  of  sediment 
per  unit  of  volume  of  water  does  not  always  increase 
with  the  velocity  of  the  river,  nor  with  the  volume  of 
flow,  but  that  a  greater  load  is  often  carried  per  unit 
of  volume  in  dry-weather  flows  than  during  floods. 
In  turbid  flowing  water,  that  near  the  surface  contains 
the  least  suspended  matter  per  unit  of  volume  of  wa- 
ter. It  is  also  apt  to  be  more  free  from  bacteria,  both 
on  account  of  the  influence  of  sunlight,  and  from  their 
being  carried  down  by  sediment.  That  the  amount 
of  sediment  is  less  at  the  surface  than  at  other  depths 
is  shown  by  numerous  recorded  observations.  It  is 
perfectly  shown  by  the  determinations  for  the 
Garonne,f  and  also  in  the  data  compiled  by  Elon 
H.  Hooker,  Ph.D.,  C.E.J 

Such  few  measurements  as  have  been  made  throw 
no  light  on  the  question  as  to  whether  more  sediment 

*  Report  to  Hon.  James  McMillan,  Chairman  Senate  Com.  on 
the  Dist.  of  Columbia,  Washington,  D.  C.,  by  Rudolph  Hering, 
George  W.  Fuller,  and  Allen  Hazen. 

t  Notice  sur  le  Port  de  Bordeaux,  M.  R.  de  Volontat,  Paris, 
1886. 

%  Trans.  Am.  Soc.  C.  E.,  vol.  xxn.  p.  414. 


$6  WATER  FILTRATION    WORKS. 

is  to  be  expected  in  the  centre  of  the  stream  than 
near  the  banks.*  So  far  as  our  present  knowledge 
goes  there  seems  to  be  but  little  difference. 

There  is  a  certain  degree  oi  comminution,  for  any 
given  material,  at  which  the  rate  of  sedimentation  of 
its  particles  in  quiet  water  would  be  so  slow  as  to  be 
practically  zero.  This  partly  explains  why  water 
which  contains  very  finely  divided  sediment  clears 
slowly,  and  also  why,  after  a  certain  period  of  time, 
practically  the  same  amount  of  clarification  will  exist 
from  the  top  to  the  bottom  of  the  water.  This  occurs 
in  the  case  of  the  water  of  the  Mississippi  at  St.  Louis. 
There  it  has  been  found  that  water  can  be  drawn 
off  from  the  settling  basins  at  the  thirteen-foot  level 
with  the  same  benefits,  as  to  clarification,  as  could  be 
had  by  drawing  it  off  from  the  top. 

Turbidity.  Standard  of  Measurement. — The  method 
devised  by  Mr.  Hazen  for  the  measurement  of  tur- 
bidity is  based  on  the  depth,  in  inches,  that  a  plati- 
num wire  i  mm.  in  diameter  and  i  inch  long  can  be 
seen  when  submerged  below  the  surface,  the  results 
being  expressed  in  the  reciprocals  of  these  depths. 
Thus,  at  a  depth  of  i  inch  the  turbidity  is  i, 
at  4  inches  it  is  .25  and  at  40  inches  .025,  etc. 
The  limit  of  permissible  turbidity  is  variously  esti- 
mated at  from  .2  to  .025,  as  water  of  this  degree 
of  clearness  will  show  no  color  to  the  ordinary  ob- 
server when  seen  through  a  glass.  The  permissible 
limit,  however,  must  depend  largely  on  the  people 
who  use  the  water,  and  turbidity  much  higher  than 

*  Report  on  the  Mississippi.     Humphreys  &  Abbott,  1861. 


SZD2M&NTA  TIOM  37 

this,  occurring  only  occasionally,  might  not  cause 
unfavorable  comment. 

Another  means  of  indicating  the  turbidity  of  a  wa- 
ter is  to  state  the  parts  per  million  of  suspended  mat- 
ter contained  therein.  This  method  was  adopted  by 
Mr.  Fuller  in  his  Louisville  and  Cincinnati  experi- 
ments. Mr.  Geo.  C.  Whipple,  Director  of  the  Mt. 
Prospect  Laboratory,  Brooklyn,  uses  silica  stand- 
ards,* prepared  from  diatomaceous  earth  and  distilled 
water  for  estimating  turbidity.  Tubes  are  filled  with 
the  mixture  diluted  by  known  quantities  of  distilled 
water,  and  the  sample  under  observation  is  compared 
with  the  various  standards  to  determine  its  turbidity. 

Rate  of  Sedimentation. — The  rate  at  which  clarifica- 
tion takes  place  in  a  quiescent  turbid  water  varies  ac- 
cording to  many  different  causes.  In  1865  Mr.  Flad 
found,  from  experiments  with  water  taken  from  the 
Mississippi  at  St.  Louis,f  that  of  a  total  of  1,000  parts 
in  suspension,  944.5  parts  settled  during  the  first  24 
hours,  22.35  parts  during  the  second  24  hours,  2.92 
parts  during  the  second  48  hours,  while  30.23  parts 
were  still  in  suspension  after  96  hours. 

Water  taken  from  the  Garonne  often  shows  tur- 
bidity after  eight  days,  and  muddy  water  taken  from 
the  lower  Elbe  shows  very  slight  sedimentation  until 
after  the  lapse  of  24  hours.  The  water  of  the  Missouri, 

*  Silica  Standards  for  the  Determination  of  Turbidity  in  Water. 
Geo.  C.  Whipple  and  Daniel  D.  Jackson.  Technology  Quarterly, 
Dec  ,  iSgq. 

f  Silt  Movement  in  the  Mississippi.  R.  E.  McMath,  Van  Nos- 
trand's  Mag.,  1883. 


WATER  FILTRATION  WORKS. 


at  Omaha,  often  refuses  to  settle  in  a  period  of  72 
hours,  while  the  waters  of  the  Delaware  and  Schuyl- 


lOOi 


10  29  30 

PERIOD  OF  QUIESCENT  SUBSIDENCE 
HOURS 

FIG.  i. — RATE  OF  SUBSIDENCE  OF  MISSISSIPPI  RIVER  WATER  AT 
ST.  Louis,  Mo. 

kill  Rivers,  at  Philadelphia,  sometimes  show  more 
matter  deposited  in  a  given  sample  of  water  at  the 
end  of  24  hours  than  in  the  same  sample  after  the 
lapse  of  48  hours. 

The  rate  of  clarification  of  the  Mississippi  River 
water,  as  determined  by  the  experiments  of  Mr.  Flad, 


SEDIMENT  A  TlON. 


39 


in  1886,  is  shown  in  Fig  i,  on  page  38.  The  curves 
represent  the  rate  of  deposition  of  sediment  in  differ- 
ent periods  of  time  for  two  sets  of  experiments.  It 
will  be  observed  that  the  amount  of  sedimentation 
which  took  place  after  the  first  24  hours  was  very  in- 
significant. The  water  in  experiment  II  cleared  more 
slowly  than  in  experiment  I,  indicating  a  more  finely 
comminuted  sediment. 

At  Cincinnati*  the  Ohio  River  contains,  at  times, 
a  sediment  so  finely  divided  that  only  75  per  cent,  of 
it,  on  the  average,  can  be  deposited  in  three  days  by 
simple  subsidence.  The  relative  estimated  range  of 
removal  of  suspended  matters,  in  different  periods  of 
time,  are  given  as  follows:  ' 

TABLE  V. 


Period  of  Subsidence. 

Percentage  Removal  of  Suspended  Matter. 

Maximum. 

Minimum. 

Average. 

24  hours        

85 
go 
95 
95 

25 
30 
40 

45 

62 
68 
72 
76 

48       " 

06      " 

Effects  of  Winds. — Experiments  made  at  St.  Louis, 
to  show  the  relative  rates  of  subsidence  of  the  water 
in  the  settling  basins  open  to  the  weather,  and  in  a 
stand-pipe  protected  from  the.  wind,  demonstrated 
that  there  was  no  practical  difference  between  the 


*  Purification  of  the  Ohio  River  Water,  for  the  Improved  Water- 
supply  of  the  City  of  Cincinnati,  O.,  1899. 


4O  WATER  FILTRATION 

two.  The  samples  from  the  stand-pipe  were  taken 
six  feet  below  the  surface,  and  those  from  the  settling 
basins  were  taken  from  the  surface  of  the  water.  On 
the  strength  of  these  indications  it  was  decided  not  to 
cover  any  settling  basins  needed  in  future  extensions 
of  the  works. 

Effect  of  Temperature. — The  influence  of  temper- 
ature on  the  rate  of  sedimentation  has  been  found  to 
be  undoubted  and  positive;  sedimentation  taking 
place  more  rapidly  in  warm*  than  in  cold  water.f 
A  difference  in  temperature  of  a  few  degrees  in  the 
water  in  different  parts  of  the  settling  basin  may  act 
as  a  disturbing  element  to  prevent  sedimentation  by 
setting  up  convection  currents  due  to  differences  in 
density.  At  St.  Louis,  in  the  Chain  of  Rocks  settling 
basins,  vortex  motion  has  been  observed  four  days 
after  filling  has  been  stopped.^ 

Effect  of  Light. — Light  is  also  a  factor  in  the  rate  of 
sedimentation,  though  its  effects  may  be  too  slight  to 
entitle  it  to  mention.  Mr.  Andrew  Brown,  in  experi- 
ments with  phials  filled  with  turbid  water,  found  that 
there  was  a  tendency  toward  more  rapid  settling  in 
those  protected  from  light  than  in  those  not  so  pro- 
tected. Not  enough  is  yet  known  about  this  subject, 
however,  to  enable  us  to  say  whether  the  phenomena 
should  influence  in  any  way  the  designing  of  settling 
basins. 

*  Subsidence  of  Fine  Solid  Particles  in  Liquids:    Carl   Barus, 
Bulletin  No.  36  U.  S.  Geological  Survey,  1886. 
f  Mass.  State  Board  of  Health,  1895,  H.  W.  Clark. 
\  Sedimentation.     James  A.  Seddon,  Eng.  News,  Dec.,  28,  1889. 


SEDIMENTATION.  4* 

Use  of  Chemicals  to  Aid  Sedimentation. — The  use  of 
chemicals  for  hastening  sedimentation  may  some- 
times be  advisable  if  the  water  contains  in  suspension 
particles  of  argillaceous,  silicious  or  earthy  matter,  so 
finely  divided  that  their  removal  cannot  be  accom- 
plished by  simple  sedimentation. 

Such  a  plan  has  been  recommended  for  Cincin- 
nati, where  it  will  be  most  advantageous  to  intro- 
duce the  sulphate  of  alumina  into  the  water  as  it 
enters  the  settling  basins,  securing  in  a  few  hours 
as  much  clarification  as  could  be  had  by  several  days 
of  simple  subsidence.  A  similar  practice  has  been 
recently  recommended  for  the  City  of  Washington, 
D.  C*  At  Sandhurst,  Victoria,  Australia,  the  water 
from  surface  gathering  grounds  contained  as  much 
as  from  24  to  32  grains  of  yellowish-brown  clayey 
matter  per  gallon,  and  filters  were  not  able  to  remove 
it.  The  addition  of  5.6  grains  of  lime  per  gallon  gave  a 
clear  water  after  10  'hours  of  settlement. 

Results  of  Sedimentation. — As  a  general  thing,  prac- 
tically all  the  suspended  matter  which  can  be  econom- 
ically removed  by  simple  subsidence  will  be  precipi- 
tated in  24  hours,  although  in  some  cases  longer  set- 
tlement may  be  more  economical  than  coagulation 
and  secondary  subsidence.  Frequently  when  certain 
waters  stand  in  reservoirs  exposed  to  the  bright  sun- 
light, they  develop  very  disagreeable  odors  and 
tastes,  the  removal  of  w'hich  requires  a  further  puri- 
fying treatment.  Such  troubles  add  considerably  to 

*  Purification  of  the  Washington  Water-supply,  Senate  Report 
2380,  $6th  Cong.,  2d  session. 


42  WATER  FILTRATION  WORKS. 

the  expense  of  purification,  necessitating,  in  some 
cases,  thorough  aeration;  in  others  filtration  and 
aeration,  and  in  others  some  chemical  or  mechanical 
treatment  to  remove  the  objectionable  qualities.  Too 
great  a  storage  capacity,  therefore,  may  sometimes 
prove  a  source  of  expense,  and  in  such  cases  it  may  be 
found  cheaper  to  remove  only  a  part  of  the  suspended 
matter  by  means  of  settling  basins,  and  to  depend 
upon  coagulation  and  secondary  subsidence,  or  upon 
filters  operated  at  a  comparatively  high  rate,  with  a 
coagulant,  for  the  removal  of  the  remainder. 

The  results  that  usually  may  be  expected,  toward 
effecting  purification  by  sedimentation,  are  the  re- 
moval under  poor  conditions  of  from  25  to  50  per 
cent.,  and  under  favorable  conditions  of  from  90  to  99 
per  cent,  of  the  suspended  matter  by  weight.  With 
the  deposition  of  the  sediment,  there  will  also  take 
place,  to  a  considerable  extent,  a  subsidence  of  some 
of  the  bacteria  in  the  water.  Examinations  made  by 
Frankland  showed  that  from  80  to  90  per  cent,  of  the 
bacteria  may  be  removed  in  this  way,  and  experiments 
made  by  Prof.  C.  C.  Brown,  in  the  St.  Louis  settling 
basins,  show  a  quite  decided  reduction  in  the  number 
of  bacteria  after  24  hours  of  settlement. 

Efficiency  of  Sedimentation. — The  relative  economy 
and  efficiency  of  the  continuous  and  intermittent 
methods  of  operating  settling  basins  are  somewhat 
disputed  points  in  this  country.  The  practice  in  Eu- 
rope inclines  toward  continuous  operation.  In  1886 
certain  experiments  on  this  subject  were  conducted 
in  St.  Louis,  under  the  direction  of  the  Water  Com- 


SEDIMENT  A  TtOtt.  43 

missioner,  Mr.  M.  L.  Holman.  At  the  time  of  these 
experiments  there  -were  in  operation  four  settling 
basins  at  the  Chain  of  Rocks  Station,  each  600  feet 
long,  270  feet  wide  and  13  feet  deep.  They  were  oper- 
ated on  the  fill-and-draw  method,  one  in  filling,  one 
in  drawing  and  two  in  settlement.  The  average 
quantity  drawn  off  at  each  drawing  was  from  10  to  12 
million  gallons.  The  daily  average  consumption  was 
about  32  million  gallons.  Thus  each  basin  had  an 
average  period  of  rest  of  from  16  to  18  'hours,  includ- 
ing the  time  of  filling  and  drawing.  The  clarified  wa- 
ter went  to  a  well  called  the  "  clear  well,"  from  which 
it  was  drawn  into  the  distribution  system.  The  ex- 
periments on  continuous  flow  were  made  with  a  flume 
2^  feet  deep,  4^  feet  wide  and  about  500  feet  long. 
The  raw  water  taken  to  the  flume  was  the  same  as  was 
taken  into  the  basins,  and  in  the  diagram  is  called  the 
"  Distributing  Well."  The  relative  clarification  was 
determined  by  an  apparatus  called  a  comparator, 
which  served  to  show  the  depth  of  the  'sample  of  wa- 
ter which  would  obscure  diffused  daylight,  and  thus 
indicate  the  degree  of  clarification.  Its  determina- 
tions while  not  exact,  and  subject  to  a  considerable 
personal  factor,  serve  as  a  fair  guide  in  judging  of  the 
results  obtained.  These  data  are  plotted  in  Fig.  2. 

In  this  diagram  is  shown  the  degree  of  clarification 
in  the  different  portions  of  the  flume,  as  the  water 
passed  through  it  at  different  mean  velocities.  The 
figures  on  the  right  are  the  comparator  readings;  the 
higher  the  number,  the  clearer  the  water.  The  in- 
clination of  each  line  then  represents  the  rate  of  clear- 


44 


WATER  FILTRATION   WORKS. 


<  60  *s  60  x  60'x  60'x  Q60' 

FIG.  2.— RATE  OF  CLARIFICATION  OF  MISSISSIPPI  RIVER  WATER 
AT  ST.  Louis,  Mo.,  IN  PASSING  SLOWLY  THROUGH  A  LONG 
FLUME. 


SETTLING  BASINS.  45 

ing.  The  diagram  shows  that  the  greater  the  velocity 
of  flow,  the  less  rapid  the  clearing,  and  the  less  abso- 
lute clearing  accomplished;  and  the  slower  the  veloc- 
ity the  more  rapid  the  clearing  and  the  more  absolute 
clarification  accomplished  in  traversing  the  flume; 
and  also  that  probably  the  effect  of  the  dams  across 
the  flume  was  detrimental  to  the  settlement  in  every 
case.  The  detrimental  effect  was  more  evident  at  the 
higher  velocities.  At  the  slow  velocities  of  1.3  and  1.4 
inches  per  minute  the  efficiency  of  the  continuous 
flow,  even  in  this  small  flume,  was  about  the  same  as 
resulted  from  16  to  18  hours  of  quiet  settlement  in  the 
large  settling  basins.  It  is  probable  that  a  velocity  of 
2.4  to  2.5  inches  per  minute  would,  allowance  being 
made  for  the  bad  effect  of  the  dams,  effect  a  clarifica- 
tion practically  equal  to  that  obtained  by  from  16  to 
1 8  hours  of  quiescent  settlement,  in  traversing  the 
500  feet  of  flume.  It  is  also  probable  that  this  effect 
might  have  been  looked  for  with  a  length  of  flume  of 
about  400  feet  if  the  dams  had  been  omitted. 

SETTLING  BASINS. 

Designing. 

Location. — Settling  basins  are  sometimes  placed  so 
low  that  the  water  from  the  river  may  be  flooded  into 
them  rapidly  by  gravity.  They  are  frequently  placed 
on  the  bank,  and  are  filled  by  pumping,  and  they  are 
also  sometimes  placed  on  a  hill,  the  water  being 
pumped  into  them,  and,  after  clarification,  allowed  to 
flow  into  the  distribution  system.  In  Antwerp  and 


46  WATER  FILTRATION   WORKS. 

Rotterdam,  situated  on  the  tidal  rivers  Nethe  and 
Maas  respectively,  the  quality  of  the  raw  water  varies 
with  the  tides,  and  the  settling  basins  have  been 
placed  sufficiently  low  to  permit  of  a  whole  day's  sup- 
ply being  rapidly  flooded  into  them  when  the  water  is 
in  its  best  condition,  without  pumping.  In  several  of 
the  London  works  also  the  settling  basins  are  lower 
than  the  rivers.  In  most  of  the  German  works,  how- 
ever, and  in  many  of  the  English,  the  basins  are  high 
enough  to  be  above  floods  and  the  raw  water  is 
pumped  into  them.  At  Altona  the  raw  water  of  the 
Elbe  is  pumped  into  the  basins  on  a  high  hill,  flows  to 
the  filters  by  gravity,  and  thence  to  the  city.  Similar 
arrangements  will  also  be  necessary  in  some  of  the 
filter  plants  now  being  built  for  the  city  of  Phila- 
delphia. 

Capacity. — The  capacity  which  should  be  given  to 
the  settling  basins  will  depend  upon  the  purpose  they 
are  intended  to  serve,  and  will  vary  from  ^  or  J  of  a 
day's  supply  of  water  to  several  days'  supply.  If  the 
river  from  which  the  supply  is  drawn  is  only  slightly 
turbid,  ordinarily,  but  is  subject  to  being  made  roily 
for  three  or  four  days  at  a  time,  by  floods  of  short  du- 
ration, a  small  capacity  only  need  be  provided.  If  the 
river  is  constantly  turbid,  and  carries  a  great  or  even 
greater  proportion  of  matter  in  suspension  during 
dry-weather  flow  than  during  floods,  like  the  lower 
Mississippi,  and  portions  of  the  Red,  Arkansas  and 
Missouri  Rivers,  a  storage  capacity  equal  to  at  least 
one  or  two  days'  supply  should  be  provided,  and  very 
frequently  chemicals  will  have  to  be  employed  to 


SETTLING  BASINS.  47 

bring  about  a  secondary  subsidence  of  the  finer  parti- 
cles. 

It  sometimes  happens,  however,  that  it  is  desirable 
to  have  a  large  storage  capacity  for  other  consider- 
ations than  those  of  economy  of  operation.  The  city 
of  London  offers  an  example  of  such  conditions.  The 
valleys  of  the  rivers  Thames  and  Lea,  above  London, 
are  quite  thickly  populated,  and  the  sewage  of  this 
population  finds  its  way  into  the  streams,  after  hav- 
ing been  treated  chemically,  or  by  its  application  to 
land;  the  treatment  is  thoroughly  carried  out,  how- 
ever, only  in  times  of  low  water.  When  the  rivers  are 
high,  a  large  amount  of  untreated  sewage  goes  into 
them,  through  storm-water  overflows,  and  by  di- 
rect discharge.  The  water  companies,  in  order  to 
secure  the  water  in  as  good  a  condition  as  possible, 
pump  it  from  the  river  to  the  settling  basins  .only 
when  the  rivers  are  low,  and  therefore  provide  suffi- 
cient storage  capacity  to  carry  them  over  periods  of 
high  water.  This  has  resulted  in  the  construction  of 
basins  very  much  larger  than  would  otherwise  have 
been  necessary;  the  different  companies  now  having 
storage  capacity  ranging  from  about  two  to  fourteen 
times  their  daily  average  consumption,  with  a  tend- 
ency to  still  further  increase  the  reserve  quantity. 

Depth. — Before  determining  the  area  required  for 
settling  basins  it  is  necessary  to  decide  upon  their 
proper  depth.  To  establish  this  point  it  will  be  neces- 
sary in  some  cases  to  make  experiments,  because 
the  deeper  it  is  possible  to  make  the  basins  the 
less  area  will  be  required.  Usually  it  will  be  found 


48  WATER  FILTRATION    WORKS. 

that  the  depth  can  be  so  great  that  the  problem  be- 
comes one  of  economically  designing  a  storage  reser- 
voir to  hold  the  given  amount  of  water.  This  was 
found  to  be  the  case  at  St.  Louis.  In  practice  the 
depth  of  water  is  usually  made  from  ten  to  sixteen 
feet,  allowing  from  two  to  three  feet  of  this  depth  for 
the  accumulation  of  sediment.  Since  in  small  basins 
the  proportionate  cost  of  the  walls  around  the 
periphery  is  greater  than  in  large  ones,  it  is  evident 
that  it  would  be  economy  to  make  small  basins  shal- 
low and  large  ones  deep.  A  less  depth  than  about  ten 
feet,  however,  would  scarcely  be  recommended. 

Length,  and  Velocity  of  Flow. — In  basins  to  be  oper- 
ated on  the  continuous-flow  method  the  first  point  to 
be  decided  is  the  proper  velocity  to  be  given  the  wa- 
ter in  its  passage  through  the  basins. 

If  the  flow  is  too  rapid,  eddies  will  be  produced 
which  will  interfere  with  the  subsidence  of  the  finer 
matter.  It  would  obviously  be  poor  economy  to  con- 
struct very  long  basins  for  a  water  which  clears  rap- 
idly, because  most  of  the  sedimentation  would  take 
place  near  the  inlet  for  raw  water,  and  the  surplus 
length  would  be  unnecessary.  In  a  water  which 
clears  very  slowly,  however,  that  is,  water  dis- 
colored largely  with  clay  or  very  finely  comminuted 
matter,  better  results  should  be  obtained  by  making 
the  basins  long  in  order  to  give  sufficient  time  for 
sedimentation. 

As  to  the  maximum  allowable  velocity,  author- 
ities differ.  Where  the  process  is  to  be  followed 
by  coagulation  or  filtration,  greater  absolute  ve- 


SETTLING  BASINS.  49 

locities  are  sometimes  allowable  than  where  sedi- 
mentation is  the  only  treatment.  Sedimentation 
alone  cannot  be  relied  on  to  produce  sufficient  clari- 
fication excepting  in  waters  which  ordinarily  run  clear 
enough  for  use  without  it.  If  the  normal  condition  of 
the  water  is  turbid,  there  is  almost  always  a  perma- 
nent discoloration  due  to  clay  and  finely  divided  or- 
ganic matter  in  suspension,  which  simple  sedimenta- 
tion alone  would  not  remove  in  many  days  of  absolute 
rest.  We  find,  therefore,  in  the  works  which  have 
been  executed,  that  the  assumed  velocity  varies  very 
greatly,  according  to  the  judgment  of  the  different 
designers.  In  Hamburg  and  Altona,  which  use  the 
turbid,  dark-colored  water  of  the  Elbe,  the  velocities 
are  about  4.5  and  3.8  inches  per  minute  respectively. 
At  both  of  these  places  the  water  is  subsequently  fil- 
tered. Professor  Freuhling*  recommends  a  velocity 
of  from  about  2.35  to  4.7  inches  per  minute. 

The  data  for  the  settling  basins  at  several  places  are 
given  in  Table  VI. 

Having  decided  upon  the  capacity  and  the  rates  of 
flow  through  the  basins,  their  lengths  may  be  found. 
In  large  works  the  determination  of  the  number  of 
basins  and  the  width  of  each  is  a  matter  of  economi- 
cally subdividing  the  total  capacity  (knowing  their 
depth,  and  length,  and  the  approximate  amount  of 
sediment  to  be  removed  in  a  given  time)  in  such  a 
manner  as  to  leave  a  sufficient  number  of  basins  al- 
ways in  operation  while  one  is  being  cleaned  or  re- 
paired. The  number  of  basins  to  be  provided  in  a  se- 
*  Handbuch  der  Ingenieurwissenschaften, 


WATER  FILTRATION    WORKS. 


ries  designed  for  the  fill-and-draw  method  should  be 
such  as  will  give  the  longest  period  of  rest  to  the 
standing  water,  regard  being  had  to  the  relative 
economy  of  construction  of  different  designs;  for  it 
must  be  borne  in  mind  that  the  interest  and  sinking- 
fund  charges  are  a  large  part  of  the  cost  of  sedimen- 
tation, being  nearly  always  more  than  the  cost  of 
operating  and  cleaning  the  basins. 

TABLE   VI. 


. 

..j 

^  « 

bo 

^*«H 

c  c 

w 

<u  ^ 

4J   f-ii  D 

s""1  *i 

City. 

Daily 
Consumption. 

Capacity  of 
Basins. 
Gallons. 

1 

"o 

rt° 

!ls 

"c  c  c 

fi? 

6 

IP 

'rt  O*o  <! 

lc-^ 

2,3IO,OOO 

I,32O,OOO 

2 

1277 

3,OOO,OOO 

6,130,312 

2 

2 

*^  /  / 
I25O 

Q3 

Baroda  

3  000,000 

16,700,000 

2 

«;  6 

i.  **3\s 

oe 

35,OOO,OOO 

84,000,000 

4" 

D  •  v 
2-4 

•767^ 

•  JO 

' 

Prof.        Frueh- 

*  •  ^f 

J^  J  J 

liners'  rec'n. 

A  O  AC\ 

2  .  34 

St    Louis     .    .  . 

75  ooo  ooo 

I57,OOO,OOO 

6 

2 

2272 

2.  50 

Vicksburg 

I,5OO,OOO 

* 

Altona,  '90  

4,900,000 

1,500,000 

2 

i 

3800 

3-8 

London  : 

11,800,000 

l68,OOO,OOO 

14.2 

E.  London.  .  . 

53,870,000 

738,000,000 

.... 

13-7 

G.  Junction.. 

22,000,000 

77,000,000 

3-5 

Lambeth  

23,538,000 

153^00,000 

.... 

6.5 

N.  River  

40,000,000 

203,000,000 

.  .  .  . 

5.1 

Southw.    and 

Vauxh  

44  OOO.OOO 

79,000,000 

1.8 

W.  Middlesex 

4f.if  ,  V  W  ,  W  W 
2O,I5O,OOO 

141,000,000 

7.0 

The  frequency  of  cleaning  will,  of  course,  depend 
upon  the  amount  of  suspended  matter  carried  by  the 
water  at  different  times  and  seasons,  on  the  water  con- 
sumption, and  on  other  factors.  At  times,  and  under 


SETTLING  BASINS.  51 

some  conditions,  basins  may  go  for  a  year  or  more 
without  cleaning  being  necessary,  or  they  may  re- 
quire it  at  intervals  of  a  few  weeks. 

Form. — The  form  to  be  given  to  the  basins  will  de- 
pend, probably,  on  the  configuration  of  the  ground. 
Where  one  shape  can  be  used  as  well  as  another,  the 
square  or  circle,  which  take  less  periphery  to  surround 
a  given  area  than  an  oblong  or  oval  shape,  may  be  ad- 
vantageous. In  large  works  basins  are  built  in  groups 
in  order  to  have  always  sufficient  storage  to  allow  of 
one  basin  being  cleaned  without  working  the  others 
at  too  high  a  rate.  They  are  usually  arranged  side  by 
side  for  the  sake  of  economy  of  construction;  the  in- 
lets for  raw  water  being  at  one  end  and  the  outlets  for 
the  settled  water  at  the  other.  At  Omaha,  Neb.,  where 
the  water  is  taken  from  the  Missouri  River,  it  was 
found  that  sometimes  clarification  would  not  follow 
sedimentation,  even  with  periods  of  rest  up  to  72 
hours.  The  question  was  said  to  have  finally  been 
satisfactorily  solved  by  causing  the  water  to  flow 
through  a  series  of  five  settling  basins  of  different 
sizes.  The  water  flows  from  each  basin  to  the  next 
over  wide,  sharp-edged  weirs,  falling  a  height  of  from 
6  to  9  inches,  in  a  thin  sheet,  by  which  means  aeration 
is  promoted,  in  order  to  counteract  the  tendency  to 
bad  odors  caused  by  the  necessarily  long  period  of 
time  occupied  by  the  water  in  passing  through  the 
basins.  This  feature  was  covered  by  patents. 

Arrangements  to  Draw  Off  Water  Longest  in  Stor- 
age.— In  some  places  the  basins  are  divided  by  longi- 
tudinal partitions  in  such  a  way  as  to  force  the  water 


52  WATER  FILTRATION    WORKS. 

to  take  a  circuitous  course  from  the  inlet  to  the  out- 
let, and  thus  insure  greater  certainty  that  the  water 
that  has  been  in  the  basin  longest  will  be  removed 
first,  and  to  prevent  the  direct  washing  of  the  water 
across  the  basin  from  the  inlet  to  the  outlet.  In 
Frankfort-on-the-Main,  Mr.  William  Lindley  has  pro- 
vided means  to  draw  off  the  water  first  that  has  been 
longest  in  storage  by  a  series  of  immersion  plates, 
partitions  extending  across  the  basins,  movable  verti- 
cally in  slots  at  each  end,  and  slightly  less  in  height 
than  the  depth  of  the  water  in  the  basins.  If  the  water 
coming  in  from  the  supply  main  is  warmer  than  that 
in  the  basins,  the  plates  are  drawn  up  so  that  the  water 
to  get  out  must  go  downward,  thus  forcing  out  first 
the  water  that  has  been  longest  in  storage.  If  the 
supply-water  is  cooler  than  that  in  the  basins,  the  im- 
mersion plates  are  forced  to  the  bottom  and  the 
water  is  drawn  off  over  their  tops. 

Locations  of  Inlets  and  Outlets. — For  continuous 
operation  the  inlet  should  be  near  the  bottom,  at  one 
end,  and  the  outlet  near  the  top  at  the  other  end.  If 
the  basin  is  very  long  the  inlet  and  outlet  might  both 
be  from  3  to  4  feet  above  the  bottom.  The  opening 
of  the  inlet  should  be  large,  or  perhaps  it  would  be 
better  to  have  several,  entering  at  points  some  dis- 
tance apart,  in  order  to  reduce  the  velocity  of  the  en- 
tering water  as  much  as  possible,  so  as  to  deposit  the 
heavy  sediment  near  the  inlets,  and  to  improve  the 
conditions  of  flow  throught  the  basin.  There  is  no 
objection  to  the  inlet  being  at  or  near  the  bottom  of 
the  basin.  The  outlet  should  be  at  least  3  or  4  feet 


SETTLING  BASINS.  53 

below  the  surface,  in  order  to  exclude  floating  objects 
and  to  avoid  the  danger  of  clogging  with  ice.  The 
successful  practice  of  to-day  indicates  that  floating 
arms  and  stand-pipes  with  many  draw-off  valves  at 
different  elevations  are  useless  refinements,  likely  to 
give  much  trouble  in  cold  climates;  it  being  generally 
perfectly  satisfactory  to  have  the  outlet  of  large  size 
some  depth  below  the  surface.  In  fact,  the  outlet  may 
be  placed  low  enough  to  be  used  on  the  fill  and  draw 
method,  if  desired.  At  the  St.  Louis  and  Ham- 
burg plants,  both  very  large,  the  outlets  are  placed  at 
but  slight  elevations  above  the  bottoms. 

Construction. 

Bottoms. — As  ordinarily  built,  after  the  excavations 
for  the  basins  have  been  made,  the  bottom  and  side 
slopes,  if  the  sides  are  not  formed  of  masonry  walls, 
are  covered  with  an  impervious  clay-puddle  carefully 
and  thoroughly  rammed  and  consolidated  to  prevent 
leakage.  The  puddle  should  vary  in  thickness  accord- 
ing to  the  character  of  the  bottom,  the  quality  and 
composition  of  the  clay,  and  the  depth  of  the  basins. 
Where  the  soil  is  firm  and  the  puddle  is  a  pure,  clean 
clay  mixed  with  about  an  equal  quantity  of  gravel,  a 
thickness  of  about  9  inches  will  suffice.  If  the  puddle 
is  made  from  a  clay  containing  a  considerable  amount 
of  micaceous  material,  a  depth  of  even  two  feet  may 
not  be  too  much,  if  the  water  has  to  be  pumped  at 
considerable  expense.  The  clay  lining  should  be 
covered  with  a  paving  of  brick,  laid  dry,  or  of  con- 
crete, in  large  slabs,  to  protect  the  clay  from  erosion, 


54  WATER  FILTRATION   WORKS. 

and  to  facilitate  cleaning.  On  the  slopes,  where  ice 
may  form,  and  frost  cause  trouble,  the  brick  paving 
should  be  bedded  in  Portland  cement,  or  the  slope- 
lining  should  consist  of  a  layer  of  strong  concrete. 

In  case  a  good  clay  for  puddle  is  not  to  be  obtained 
without  great  expense,  it  may  be  necessary  to  use  a 
lining  of  concrete  6  to  9  inches  thick  for  the  bottom 
and  slopes.  This  has  frequently  been  done  with  suc- 
cess. There  is,  however,  great  likelihood  that  such 
large  surfaces  of  concrete  may  crack  under  tem- 
perature changes  when  the  basins  are  emptied  for 
cleaning  and  the  bottom  and  slopes  exposed  to  the 
hot  sun  for  considerable  periods  of  time.  The  danger 
from  such  cracking  is  not  always  great,  but  in  case 
the  subsoil  when  wet  is  of  a  nature  to  yield  under 
pressure  considerable  settlement  may  take  place 
along  the  cracks,  and  leaks  of  serious  magnitude  may 
follow.  If  the  subsoil  when  wet  is  very  firm  and  re- 
tentive such  a  danger  would  not  be  great,  as  the 
cracks  might  possibly  silt  themselves  up  again  to  a 
condition  of  water-tightness. 

Instead  of  using  such  expensive  methods  in  the 
construction  of  settling  basins,  it  may  sometimes  be 
satisfactory  to  form  them  along  the  bank  of  the  river 
or  lake  from  which  the  water  is  taken,  by  excavating 
the  interior  space  with  dredges  and  forming  the  em- 
bankment along  the  river  side  from  the  excavated 
sand  or  earth.  The  faces  of  the  slopes,  inside  and 
outside,  should  be  protected  with  a  thick  riprap  of 
broken  stone  to  prevent  abrasion,  and  the  necessary 
inlets  and  outlets  for  the  regulation  of  the  flow 


SETTLING  &AS1MS.  $$ 

through  the  basins  should  be  provided.  Such  basins 
might  be  cleaned  at  small  expense  by  means  of  a 
suction  pump,  mounted  on  a  barge.  This  plan  was 
contemplated  in  the  settling  basins  proposed  for  the 
Torresdale  filter  plant  on  the  Delaware  River  at 
Philadelphia.* 

Under  drainage. — In  cases  where 'the  bottoms  of  the 
basins  are  lower  than  the  water  in  the  river,  it  may,  if 
the  land  is  porous,  be  necessary  to  underdrain  the 
site.  The  drains  should  discharge  into  a  sump,  from 
which,  in  wet  seasons,  the  water  may  be  pumped,  to 
prevent  the  possible  breaking  in  of  the  lining,  by  the 
upward  pressure  of  the  ground-water  when  the  basins 
are  emptied  for  cleaning.  If  two  adjoining  basins 
are  separated  by  a  division  wall  of  masonry,  great 
care  should  be  exercised  in  the  placing  of  the  puddle. 
This  should  be  of  increased  thickness  on  each  side  of 
the  wall,  and  should  extend  down  the  sides  of  it  and 
under  the  footings,  unless  the  latter  be  founded  on 
rock  or  other  impervious  material.  This  precaution 
is  necessary  to  prevent  the  blowing  out  of  the  bottom 
of  a  basin  when  its  neighbor  is  emptied  for  cleaning 
or  repairs.  An  accident  of  this  kind  happened  several 
years  ago  to  the  St.  Louis  settling  basins. 

Sides. — If  the  sides  of  the  basin  are  to  be  of  ma- 
sonry they  should  be  designed  according  to  the  ordi- 
nary rules,  as  retaining  walls,  considering  the  basin 
empty.  If  the  sides  are  simply  the  dressed  faces  of 

*  Report  on  the  Extension  and  Improvement  of  the  Water- 
supply  of  the  City  of  Philadelphia,  1899.  Rudolph  Hering, 
Joseph  M.  Wilson,  and  Samuel  M.  Gray. 


56  WATER  FILTRATION- 

the  excavation  or  embankments,  they  should  have  a 
slope  of  about  2  horizontal  to  i  vertical,  excepting  in 
very  loose  soils,  when  the  slope  should  be  increased 
to  3  to  i. 

Arrangements  for  Cleaning. — The  bottom  of  each 
basin  should  have  a  longitudinal  channel  through  the 
centre,  sloping,  as  circumstances  may  make  neces- 
sary, toward  one  end  or  the  other,  with  a  slope  of 
about  i  in  500.  The  bottom  surface  should  slope 
toward  this  channel  with  an  inclination  of  about  i  in 
200,  in  order  to  facilitate  the  removal  of  the  sedi- 
ment. If  the  basins  are  very  large,  and  construction 
expensive,  flatter  slopes  than  these  may  be  used.  At 
the  large  basins  at  Hamburg  the  bottom  slope  longi- 
tudinally is  only  about  i  in  1750.  At  Omaha,  Neb., 
the  bottoms  of  the  basins,  instead  of  being  sloped  in 
only  two  directions,  are  formed  of  a  series  of  de- 
pressions, toward  which  the  sludge  gravitates  or  may 
be  pushed.  The  sludge  is  taken  to  the  clean-out  con- 
duit through  a  mud-valve  at  the  lowest  point  of  each 
depression.  A  system  of  four-inch  water-mains,  with 
convenient  hose  connections,  supplies  the  water  for 
washing  out  the  basins. 

Regulating  Apparatus. — At  Hamburg  the  water  of 
the  Elbe  is  pumped  into  a  large  channel,  which 
supplies  all  the  basins.  As  the  basins  lie  between  this 
channel  and  the  filters  it  is  necessary  to  regulate  both 
the  inflow  of  water  to  the  basins  and  the  outflow  to 
the  filters  in  order,  first,  that  the  basins  may  not  be 
overflowed,  and,  second,  that  the  too  rapid  flow  of 
clarified  water  to  the  filters  may  not  cause  the  water 


PLATE  III. — SETTLING  BASIN,  ALBANY,  N.  Y.  VIEW  SHOWING 
INLET  FOR  RAW  WATER;  SLOPE  PAVING;  CONCRETE  BOTTOM, 
AND  METHOD  OF  REMOVING  SEDIMENT. 

57 


SETTLING  BASINS.  59 

to  stand  too  deeply  on  the  surface  of  the  filters.  Each 
basin  is  filled  through  two  branch-pipes,  about  3  feet 
in  diameter  and  at  right  angles  in  a  horizontal  plane, 
which  join  in  a  cast-iron  vertical  cylinder  about  4  feet 
3  inches  in  diameter  in  the  gate-house.  The  water 
from  the  canal  is  admitted  to  this  cylinder  by  raising 
a  double-seated  valve,  and  then  flows  into  the  basin 
through  the  branch-pipes.  When  the  water  stands  at 
the  proper  height  in  the  basin  the  valve  is  closed. 

The  water  flows  to  the  filters  through  a  regulating 
house  at  the  opposite  end  of  the  settling  basin,  in 
which  is  placed  a  double-seated  valve  operated  by 
a  float  resting  upon  the  surface  of  the  water  in  the 
canal  leading  to  the  filters.  This  regulates  the 
amount  of  water  flowing  to  the  filters  in  accordance 
with  the  amount  needed.  The  water  enters  the 
house  through  18  rectangular  holes  in  the  wall,  about 
three  feet  above  the  bottom.  The  regulating  valve 
can  also  be  operated  by  hand. 

The  basins  have  side  slopes  of  3  horizontal  to 
i  vertical.  They  were  built  upon  marshy  ground 
and  are  underlaid  with  a  thick  layer  of  clay-puddle. 
The  bottoms  and  sides  are  paved  with  brick,  while 
concrete  is  employed  for  protecting  the  slopes  in  the 
zone  where  ice  forms. 

At  Albany,  New  York,  the  inlets  are  perforated 
with  small  holes  above  the  water  line  of  the  basin, 
to  promote  the  aeration  of  the  water  as  it  enters  the 
basin.  This  is  shown  quite  clearly  in  Plate  III. 

In  the  English  practice  the  inlet  is  frequently  a  bell- 
mouth  pipe,  delivering  the  water  at  or  near  the  bot- 


60  WATER  FILTRATION   WORKS. 

torn  of  the  basin,  and  the  outlet  merely  a  pipe  con- 
trolled by  a  valve.  Sometimes  the  outlet  will  consist 
of  a  stand-pipe  with  several  valves  at  different  eleva- 
tions, or  of  a  floating  pipe,  one  end  of  which  is  main- 
tained at  a  certain  depth  below  the  surface  by  a  float. 

Removal  of  Sediment. — If  the  basins  are  placed  at 
such  an  elevation  that  they  can  be  drained  by 
gravity  the  removal  of  the  sediment  is  easily  ac- 
complished by  washing  and  pushing  it  toward  the 
outlet  at  one  end.  This  outlet  should  be  large  and 
should  be  closed  by  a  penstock  or  sluice-valve  oper- 
ated from  above  by  a  spindle. 

If  the  basins  are  to  be  operated  on  the  continuous 
plan,  it  would  be  preferable  to  slope  the  bottom 
downward  toward  the  inlet  end  of  the  basin  and  lo- 
cate the  outlet  for  sediment  near  the  inlet.  In  or- 
der to  facilitate  the  removal  of  the  sediment  it  might 
be  advantageous  to  provide  a  supply-main,  with  wa- 
ter under  pressure,  along  the  opposite  end  of  the 
basin  from  the  inlet,  provided  with  valves  and  nipples 
for  discharging  water  into  the  basin  at  the  upper  end, 
and  thus  supplying  a  current  for  the  removal  of  the 
sediment  by  water-carriage.  If  the  basins  are  to  be 
operated  on  the  fill-and-draw  method  the  outlets  for 
sediment  might  be  preferably  located  on  the  opposite 
end  of  the  basins  from  the  inlets  for  raw  water,  and 
the  bottom  should  then  slope  toward  the  outlet.  In 
this  case  the  necessity  does  not  exist  for  the  extra 
supply-main  for  washing  out  the  basins,  as  the  raw 
water  may  be  used  for  that  purpose.  A  very  con- 
venient arrangement  for  washing  out  the  sediment 


SETTLING  BASINS.  6 1 

is  in  use  in  St.  Louis.  It  consists  of  a  movable  siphon 
by  which  a  stream  can  be  siphoned  out  of  a  full  basin 
for  washing  a  contiguous  empty  one.  The  siphon  is 
moved  along  as  the  washing  progresses.  Other 
means  have  to  be  provided  for  the  basins  on  the  ends 
of  the  series. 

If  the  basins  are  placed  so  low  that  they  cannot  be 
drained  by  gravity,  as  at  Antwerp,  Shanghai,  Rotter- 
dam and  at  some  of  the  London  works,  it  will  be 
necessary  to  construct  in  each  basin  a  sump  to  which 
the  sediment  may  be  washed,  and  from  which  it  may 
be  removed  by  centrifugal  or  other  pumping  ma- 
chinery. 

When  the  outfall  end  of  the  clean-out  conduit  is 
subject  to  submersion  by  floods  or  tides  a  tide-flap 
should  be  placed  over  the  end  to  protect  it  from 
silting  up  during  periods  of  high  water. 

Roofing. — There  is  no  necessity  in  this  country  for 
roofing  over  large  settling  basins.  The  only  reasons 
which  can  be  urged  for  such  a  practice  would  be  to 
prevent  the  formation  of  ice,  to  protect  them  from  the 
winds,  and  from  light.  Basins,  as  usually  built,  are  of 
such  depth  that  the  formation  of  ice  will  not  cause 
serious  inconvenience;  the  reduction  of  their  capa- 
city, by  the  ice,  will  not  be  significant,  because,  dur- 
ing cold  months,  the  draft  will  generally  be  light,  and 
the  water  will,  as  a  rule,  contain  less  suspended  mat- 
ter than  in  the  summer  months.  The  effect  of  the  wind 
on  the  rate  of  sedimentation  will  be  very  slight,  as 
the  action  of  waves  in  causing  eddies  below  the  sur- 
face is  quite  insignificant,  as  a  rule,  in  such  small  areas 


62  WATER  FILTRATION   WORKS. 

and  in  waters  of  such  small  depth.  If  it  should  be 
found,  however,  that  there  was  a  retardation  of  sedi- 
mentation, or  that  the  waves  were  apt  to  damage  .the 
walls  or  slopes  of  the  basins,  the  trouble  could  easily 
be  remedied  by  a  series  of  floating  spars  resting  upon 
the  surface  of  the  water.  This  expedient  was  resorted 
to  with  success  in  the  very  large  sewage  precipitation 
tanks  at  Manchester,  England,  which  are  in  a  posi- 
tion exposed  to  very  severe  winds. 

The  necessity  of  protecting  basins  from  the  light 
vanishes  if  their  storage  capacity  does  not  exceed  a 
day  or  two's  supply;  if  objectionable  odors  or  growths 
of  algae  should  result,  on  longer  storage,  a  simple 
treatment  by  aeration,  before  storage,  might  be  suf- 
ficient to  remove  the  objectionable  qualities.  As  to 
whether  the  benefits  arising  from  covering  the  set- 
tling" basins,  due  to  maintaining  the  water  at  a  higher 
temperature  in  winter,  and  thus  favoring  more  rapid 
sedimentation,  would  offset  the  increased  cost  of 
construction,  and  also  of  operation,  it  may  be,  in  the 
light  of  our  present  experience,  answered  in  the  neg- 
ative. 

Cost. — The  cost  of  sedimentation  basins  depends 
upon  so  many  local  conditions  and  circumstances, 
that  a  comparison  of  the  data  of  different  basins 
would  give  a  very  wide  range  of  costs.  Basins  of 
about  3,000,000  gallons  capacity,  with  concrete  bot- 
toms, 12  inches  thick,  on  12  inches  of  puddle,  and 
with  concrete  sidewalls,  including  the  iron  work, 
masonry,  intake  well  and  connections,  but  excluding 
the  cost  of  land,  will  cost  not  far  from  $9,000  per 


SETTLING  BASINS.  63 

million  gallons  of  capacity.  Those  in  which  the  bot- 
toms and  sides  are  puddled  with  clay  and  paved  with 
brick  will  probably  cost  less  than  this;  and  others  in 
which  the  bottoms  are  of  concrete,  laid  on  a  puddle- 
bed,  with  asphalted  joints,  may  cost  more.  The  aver- 
age actual  cost  of  the  reservoirs  of  the  Philadelphia 
water-supply  has  been  about  $4,051  per  million  gal- 
lons of  capacity,  ranging  from  about  $3,300  to 
$4,300. 

Operation. 

The  water  should  be  taken  into  the  basins  when 
in  its  best  condition.  In  tidal  streams,  as  already 
noted,  there  will  generally  be  times  when  the  wa- 
ter will  be  more  pure  than  at  others.  Therefore,  un- 
der these  circumstances,  the  basins  should,  if  possible, 
be  so  arranged  and  placed  that  they  may  be  flooded 
very  rapidly,  and  the  intake  and  conduits  should 
be  correspondingly  large.  Where  there  is  no  appar- 
ent change  of  quality  in  the  water,  due  to  a  periodi- 
cally recurring  cause,  the  water  may  be  taken  from 
the  river  from  near  the  surface. 

Rate  of  Flow. — If  the  settling  basins  are  operated 
continuously,  local  conditions  must  determine  how 
the  rates  of  flow  should  be  regulated.  Where  the 
basins  are  joined  to  a  common  conduit,  leading  to 
pumps,  the  regulation  of  inflow  and  outflow  can 
safely  be  effected  by  the  attendants.  Their  duty  in 
this  case  would  be  to  see  that  certain  maximum  and 
minimum  depths  of  water  in  the  basins  and  conduits 
were  not  passed,  and  that  the  rate  of  delivery  from 
each  basin  did  not  exceed  the  limit  determined  upon 


64 


WATER  FILTRATION    WORKS. 


in  the  design.  The  fluctuating  draft  from  the  city, 
varying  with  the  seasons,  days  and  times  of  day, 
makes  the  work  thrown  upon  the  basins  very  variable 
unless  the  water  goes  to  storage  reservoirs  before 
being  delivered  into  the  mains.  Where  sedimenta- 
tion is  not  to  be  followed  by  filtration  it  is  not  often 
that  this  could  be  the  case,  as  in  such  works  the  ba- 
sins themselves  must  provide  the  storage  to  meet  this 
fluctuation. 

The  labor  necessary  to  effect  the  regulation  for 
the  fill-and-draw  method  amounts  in  large  plants 
to  about  25  cents  per  million  gallons.  For  the 
continuous-flow  method  it  is  less  than  half  of  this,  as 
it  may  be  done  by  automatic  apparatus  similar  to 
that  already  described  as  being  in  use  in  Hamburg. 

Amount  of  Sediment  to  be  Expected. — The  amount  of 
matter  that  will  subside  from  a  turbid  water  is  very 
difficult  to  estimate,  analyses  even  affording  but  little 
guide  as  to  what  may  be  expected.  This  may  be  seen 
from  the  data  compiled  in  Table  VII. 

TABLE   VII. 


River  and  Place  of  Observation. 

Cu.  yds. 
Sediment 
per  Million 
Gallons. 

Observer. 

Date. 

Mississippi,  St.  Louis  —  . 
'             Helena 

Q.I 

3-3 

McMath. 
Miss.  River  Comm. 

1879 
80-81 

'             Hannibal  .... 
'              Prescott  

•y 
.82 
.60 

Miss.  River  Comm. 
Clarence  Delafield 

1079 
1880 

80-8  1 

'             Clayton     .... 

18 

80-8  1 

Sacramento  River 

Garonne,  Bordeaux  

6  20 

M    R    de  Volontat 

1079 

187-1 

CQ 

l87C 

Mississippi  R.,  Vicksburg. 

5-20 

C.  R.  McFarland. 

lo/:> 
1895 

SETTLING   BASINS. 


Table  VIII  shows  the  analyses  of  the  Garonne 
waters  at  Bordeaux.  The  quantities  are  averages  for 
each  month  from  1870  to  1874,  as  determined  from 
samples  taken  from  the  surface  of  the  river  at  high 
tide. 

^ TABLE   VIII. 


Month. 

Cu.  ft.  of  Sediment 
per  Million  Gals. 

Month. 

Cu.  ft.  of  Sediment 
per  Million  Gals. 

31    26 

Tulv.. 

4Q    26 

31  68 

August        .... 

IO4   7C 

March  

18  32 

September  .... 

171    IQ 

April  

17   QI 

October  

14.7  .  80 

May  

16  .07 

November  .... 

8l  .  "?1 

Tune  

17.  eg 

December  

25  -70 

The  amount  of  sediment  removed  yearly  by  man- 
ual labor,  from  1884  to  1895,  from  the  settling  basins 
at  St.  Louis,  is  given  in  Table  IX. 
TABLE  IX. 


Year. 

Cu.  yds.  of  Sedi- 
ment Removed 
from  Basins. 

Millions  of 
Gallons  Pumped 
to  Basins. 

• 

Cu.  yds.  of  Sedi- 
ment per  Million 
Gallons. 

Cost  of  Remov- 
ing Sediment. 

1884-5 

153,000 

9,564 

15-997 

$3276.00 

1885-6 

109,000 

9,925 

10.982 

2195.36 

1886-7 

124,000 

10,979 

11.294 

2137-50 

1887-8 

143,000 

11,665 

12.336 

1865.41 

1888-9 

I74,OOO 

11,644 

14-943 

2869.60 

1889-90 

144,750 

",939 

12.124 

1386.00 

1890-1 

207,800 

13,178 

15-768 

I4II.2O 

1891-2 

210,600 

14,602 

14.423 

2286.40 

1892-3 

l6o,000 

16,448 

9-725 

2810.88 

1893-4 

148,000 

17,448 

8.482 

35I9-20 

1894-5 

208,000 

16,257 

12.794 

2668.20 

The  actual  quantity  of  suspended  matter  removed 
by  subsidence  has  been  from  three  tenths  to  four 
tenths  in  excess  of  these  quantities, 


66 


WATER  FILTRATION    WORKS. 


Depth  of  Sediment  to  be  Provided  for. — The  depth  of 
sediment  which  should  be  allowed  to  collect  in  the 
basins  before  cleaning-  them  will  vary  according  to 
the  nature  of  the  sediment  and  the  design  of  the 
basins.  If  the  sediment  is  very  heavy  the  basins  may 
require  more  frequent  cleaning  than  if  it  is  of  lighter 
specific  gravity.  At  Altona  it  is  reported  that  two 
feet  of  sediment  collected  in  the  basins  in  three 
months.  At  Hamburg  provision  is  made  for  a  depth 
of  about  three  feet  of  sediment,  which  here  is  of  light 
specific  gravity.  At  Antwerp  about  one  foot  in  depth 
is  allowed  for. 

Periods  of  Cleaning. — Table  X,  compiled  from  the 
annual  reports  of  the  Water  Commissioner  of  St. 
Louis,  illustrates  some  of  the  practical  considerations 
that  govern  the  times  of  cleaning  the  basins  at  that 
city. 

TABLE   X. 


No 

.  i. 

No 

2. 

No 

•  3- 

No 

•  4. 

a 

b 

a 

b 

a 

b 

a 

b 

April    1893  

•*6 

24 

$  165  oo 

July  

CQ 

oe 

C72    2O 

August  
September  

10 

7 

66 

48 

66 

48 

13 

C<2 

9 

•1Q 

1296.00 
418    80 

October  

^6 

24 

20 

12 

312  .  80 

IO 

6 

3O 

12 

18 

IO 

227    2O 

March    1894  

14 

g 

20 

IO 

2O 

IO 

14 

8 

C27    2O 

May 

d6 

24 

46 

24 

46 

24 

4O 

20 

781    60 

Tulv.. 

7O 

4O 

78 

48 

78 

46 

78 

46 

74^   4O 

CQ 

•5Q 

CA 

•36 

£4 

^6 

tA 

16 

864   80 

March    1805        • 

18 

18 

20 

IO 

278    40 

294 

177 

,i. 

193 

304 

186 

306 

192 

6187.40 

Column  a  gives  total  depth  in  inches  of  sediment  in  basin. 
Column  b  gives  depth  of  sediment  removed  by  manual  labor. 


SETTLING  BASINS.  6/ 

The  total  amount  of  sediment  collected  from  April, 
1893,  to  April,  1894,  was  222,000  cubic  yards,  of 
which  148,000  cubic  yards  were  removed  by  manual 
labor.  For  the  year  1894-5  the  figures  were  356,000 
and  208,000  respectively. 

A  study  of  this  table  shows  that  the  greatest 
amount  of  sediment  is  collected,  and  therefore  the 
most  frequent  cleaning  is  needed  in  the  summer 
months,  from  March  to  October.  The  maximum 
rate  of  precipitation  occurs  about  July,  which  also 
corresponds  in  general  to  the  periods  of  high  water, 
and  to  the  time  of  the  year  when  the  water  consump- 
tion is  greatest.  From  October  to  March  the  river 
is  low,  excepting  for  the  flashy  rise  due  to  the  Jan- 
uary thaws,  the  consumption  of  water  is  below  the 
average  and  the  amount  of  sediment  collected  very 
small.  The  times  of  cleaning,  therefore,  beginning 
with  the  month  of  March,  are  at  intervals  of  two 
months,  two  months,  three  months  and  five  months. 

Amount  of  Water  Necessary  for  Cleaning. — The 
amount  of  water  necessary  for  cleaning  out  a  basin 
will  depend  upon  the  slope  of  the  bottom,  the  nature 
of  the  sediment,  the  judgment  of  the  men,  and  the 
manner  in  which  the  cleaning-water  is  used.  The 
work  of  removing  the  sediment  is  done  partly  by  the 
water  and  partly  by  hand. 

With  a  certain  cross-section  of  channel  in  the  cen- 
tre of  the  basins  and  a  given  slope,  it  is  possible  to 
move  only  a  certain  quantity  of  sediment  in  the  water 
used  for  washing.  Basins  in  which  the  bottoms  are 
comparatively  flat  will,  therefore,  take  a  greater 


68  WATER  FILTRATION    WORKS. 

quantity  of  water  and  a  longer  time  to  clean  than 
those  in  which  the  inclination  is  greater.  There  are 
no  available  published  data  concerning  the  absolute 
amount  of  suspended  matter  that  can  be  carried  in 
water  flowing  in  open  troughs  or  channels  of  different 
dimensions  and  at  different  slopes.  Probably,  the 
more  finely  comminuted  the  matter,  the  greater  abso- 
lute weight  of  it  can  be  carried  by  the  water  at  any 
given  velocity.  The  removal  of  the  sediment  by  wa- 
ter-carriage is  brought  about  by  two  agencies — the 
power  of  the  water  to  carry  a  part  of  the  matter  in 
suspension  and  its  power  to  roll  on  the  bottom  parti- 
cles larger  than  it  can  carry  in  suspension.  Both  of 
these  effects  are  produced  at  a  los.s  of  energy.  Veloc- 
ities of  flow  in  the  clean-out  conduit,  calculated  ac- 
cording to  the  usual  rules,  will,  therefore,  be  too  great. 
If  the  slope  of  the  conduit  were  proportioned  so  that 
the  velocity  for  clean  water  would  be  from  about  10 
to  12  feet  per  second  when  running  half  full,  it  is  prob- 
able that  the  sediment  could  be  carried  through  it 
without  too  great  an  allowance  of  flushing-water. 
This  velocity  would  probably  carry  off  a  sandy  sedi- 
ment to  the  extent  of  about  5  per  cent,  of  the  volume 
of  the  wash-water.  If  the  sediment  were  more  earthy, 
possibly  as  much  as  10  per  cent,  could  be  carried  out. 
This  would  be  of  the  consistency  of  the  average  sew- 
age-sludge resulting  from  chemical  deposition. 

The  amount  of  sediment  carried  in  flowing  rivers  is 
very  variable.  Such  measurements  as  have  been  re- 
corded show  a  range  of  from  about  one  two-hun- 


I  ? 


- 


E 

jc     z 

Q 

tc 


•'    SETTLING  BASINS.  7 1 

dredths  of  i  per  cent,  to  about  i  per  cent,  of  the  vol- 
ume of  flowing  water. 

The  amount  of  water  used  in  washing  the  sediment 
out  of  the  basins  will,  therefore,  probably  amount  to 
from  10  to  20  times  the  volume  of  sediment  removed. 

Methods  of  Cleaning. — The  cleaning  is  done  by  the 
action  of  flowing  water,  combined  with  labor,  both  so 
directed  that  the  sediment  is  pushed  and  washed  into 
the  central  channel  and  finally  into  the  clean-out  con- 
duit. The  expense  of  cleaning  is  therefore  divided 
into  two  parts — wages  of  laborers  and  cost  of  wash-  5 
water — because  of  the  water  having  to  be  pumped.  ^ 
The  expense  of  pumpage  is  present  in  every  case  but 
one.  If  the  basins  are  placed  lower  than  the  river,  so 
that  the  water  flows  into  them,  the  expense  of  pump- 
ing falls  on  the  sediment-laden  water,  which  must  be 
removed  from  the  basins.  The  only  case  where  the 
expense  of  pumping  can  be  avoided  entirely  is  where 
the  settling  reservoirs  are  located  at  the  site  of  a  fall, 
or  dam  of  considerable  height,  so  that  the  water  may 
be  flooded  into  the  basins  and  the  wash-water  allowed 
to  flow  out  by  gravity.  This  is  a  condition,  however, 
that  will  rarely  be  realized. 

The  method  of  cleaning  the  settling  basins  at  Al- 
bany, N.  Y.,  is  illustrated  in  the  photographic  views, 
Plates  III  and  IV. 

In  cleaning  the  St.  Louis  basins  the  upper  semi- 
fluid part  of  the  sediment,  about  three  tenths  to  four 
tenths  of  the  total  amount  in  the  basins,  goes  out 
without  the  necessity  of  manual  labor  in  its  removal. 
The  remaining  six  tenths  to  seven  tenths  is  removed 


WATER  FILTRATION    WORKS. 


partly  by  water  and  partly  by  being  pushed  to  the 
outlet  with  squeegees. 

Cost  of  Removing  Sediment. — The  cost  of  removing 
the  sediment,  per  million  gallons  of  water,  will  vary 
with  the  seasonal  changes  in  the  character  of  the  raw 
water.  Estimates  of  this  sort,  therefore,  are  difficult 
to  make  and  can  serve  only  as  a  rough  approxi- 
mation. Mr.  Wm.  H.  Lindley  gives  the  cost  of  subsi- 
dence in  covered  reservoirs,  in  Germany,  including 
interest  and  sinking  fund,  at  from  $1.80  to  $2.25  per 
million  gallons,  of  which  from  50  to  60  per  cent,  is  for 
interest  and  sinking  fund.  Taking  the  values  given 
in  the  reports  of"  the  Water  Commissioner  of  St. 
Louis  for  1894  and  1895,  the  cost  per  cubic  yard  of 
sediment  removed  from  the  basins,  for  maintenance 
and  cleaning,  for  the  two  years  would  be  as  given  in 
Table  XL 

TABLE   XI. 


1894 

1895 

• 

Cents  pe 

r  cu.  yd. 

Pav-roll    gatemen    foremen    etc    .  .  .  . 

2   I 

I   J. 

Pay-roll    labor  cleaning  basins  

1.6 

3 

2  .  a 

2.8 

Water  used  in  cleaning  (estimated  by  author)  

I.O 

I.O 

Total  

70 

6  o 

Since  1884  the  average  amount  of  sediment  re- 
moved from  the  water  has  been  about  12.5  cubic 
yards  per  million  gallons,  which,  at  7  cents  per  cubic 
yard,  would  make  the  cost  about  87.5  cents  per  mill- 


SETTLING  BASINS.  73 

ion  gallons,  exclusive  of  interest  and  sinking-fund 
charges.  The  charges  for  these  items,  per  million 
gallons,  will  depend  upon  the  amount  of  water  clari- 
fied, that  is,  on  the  speed  with  which  the  water  is 
passed  through  the  basins.  As  they  are  now  oper- 
ated, on  the  fill-and-draw  method,  this  charge  cannot 
be  much  less  than  about  $3.00  per  million  gallons.  It 
must  be  remembered,  however,  that  the  basins  have 
a  capacity  for  two  days'  supply  at  maximum  draft, 
and  that  if  the  consumption  should  greatly  increase, 
the  same  basins  would  serve  without  it  being  neces- 
sary to  increase  their  capacity,  although  certain 
changes  in  details  might  be  advisable  to  reduce  the 
cost  of  operation.  When  operated  to  their  maximum 
capacity,  the  total  average  cost  of  removing  the  sedi- 
ment might  be  from  $2.25  to  $2.50  per  million  gal- 
lons of  water  passed  through  the  basins. 

Relative  Advantages  of  Fill-and-Drazv  and  Continu- 
ous Operation. — The  principal  advantage  of  the  fill- 
and-draw  method  is  that  a  more  perfect  quiescence  of 
the  water  may  be  obtained,  and  for  this  reason,  per- 
haps, a  greater  quantity  of  suspended  matter  may  be 
precipitated  in  a  given  time.  This  advantage  has  not, 
however,  in  many  works  which  have  been  executed, 
been  found  sufficiently  great  to  counterbalance  the 
disadvantages  in  point  of  cost  of  construction  and 
cost  of  operation.  If  the  sediment  is  heavy,  and  set- 
tles rapidly,  the  greatest  bulk,  under  the  continuous 
method,  will  subside  near  the  point  where  the  raw  wa- 
ter enters  the  basin.  Under  the  fill-and-draw  method 
the  sediment  will  be  spread  further  away  from  the 


74  WATER  FILTRATION  WORKS. 

inlet-pipes  by  the  currents,  as  the  basin  fills.  There- 
fore, if  the  clean-out  conduit  from  the  basin  must  be 
located  near  the  inlet  for  raw  water,  the  continuous- 
flow  method  will  deposit  the  sediment  where  it  will 
cost  less  for  its  removal  than  the  fill-and-draw  method. 
On  the  contrary,  if  the  clean-out  conduit  is  on  the  op- 
posite side  of  the  basin  the  fill-and-draw  method 
will  be  the  most  economical  in  point  of  cleaning. 
If  the  construction  of  the  settling  basins  is  pre- 
paratory to  a  process  of  filtration,  still  other  consider- 
ations may  have  weight  in  their  design.  Local  con- 
ditions will  then  have  to  decide  whether  their  opera- 
tion should  be  continuous  or  on  the  fill-and-draw 
plan.  If  the  basins  can  be  placed  higher  than  the  fil- 
ters, it  may  be  advantageous  to  use  the  fill-and-draw 
system;  if  not,  it  will  generally  be  necessary  to  oper- 
ate them  continuously,  because,  even  if  pumping  has 
to  be  resorted  to  between  the  basins  and  the  filters,  it 
will  be  on  the  side  of  economy  to  reduce  the  lift  on 
the  pumps  as  much  as  possible. 


CHAPTER  III. 

THE   PURIFICATION    OF   WATER   BY   SLOW   SAND- 
FILTRATION. 

INTRODUCTION. 

Types  of  Filters  Used  for  Filtration  of  Municipal  Sup- 
plies.— Filtration,  in  the  sense  in  which  the  word  is 
used  in  this  work,  has  for  its  object  the  removal  from 
water  of  objectionable  polluting  matter  that  cannot 
be  economically  taken  out  by  simple  subsidence,  or 
by  chemical  treatment.  The  successful  filtration  pro- 
cesses for  purifying  the  water-supplies  of  cities  and 
towns  may  be  separated  into  three  classes.  The  dis- 
tinctive characteristics  of  these  classes  are  as  follows: 
In  one,  first  adopted  in  England,  the  water  is  filtered 
slowly  through  beds  of  sand;  filters  of  this  type  are 
called  English  Filters,  Slow  Filters  or  Slow  Sand-fil- 
ters. The  second  type,  a  distinctively  American  in- 
vention, filters  the  water  rapidly  through  beds  of 
sand,  a  coagulant  having  first  been  added  to  the  wa- 
ter; filters  of  this  kind  are  called  American  Filters, 
Mechanical  Filters  or  Rapid  Sand-filters.  The  third 
type  filters  the  water  through  ^a  strainer  of  fine  mesh, 
such  as  porcelain,  concrete  slabs,  etc.  All  these  meth- 
ods are  in  use.  For  the  sake  of  uniformity,  in  the  pres- 
ent work  the  terms  Rapid  and  Slow  Sand-filters  will 

75 


76  WATER  FILTRATION 

be  used  in  referring  to  the  first  two  types,  because 
they  are  short,  distinctive  and  sufficiently  exact. 

Slow  Sand-filtration. — The  process  of  slow  sand- 
filtration  consists  of  passing  the  water  downward  by 
gravity  through  beds  of  sand  of  certain  depth,  and 
with  certain  restrictions  as  to  velocity  and  manipula- 
tion that  experience  has  shown  to  be  necessary.  By 
this  process  most  of  the  suspended  matters  in  the 
water,  including  nearly  all  of  the  bacteria,  are  re- 
tained upon  the  surface  of  the  sand;  most  of  the  re- 
maining bacteria  are  destroyed  in  the  top  layers  of 
the  filter,  while  a  portion  of  the  dissolved  organic 
matter  in  the  water  is  converted,  by  chemical  action, 
into  inorganic  compounds. 

Rapid  Sand-filtration. — The  process  of  rapid  sand- 
filtration  consists  of  passing  the  water  downward  at 
a  rapid  rate  through  small  beds  of  sand,  a  certain 
amount  of  coagulating  material  having  been  first  in- 
troduced into  the  water  to  assist  in  forming  a  scum 
on  the  surface  of  the  sand  and  a  film  between  the 
grains  of  sand  in  the  bed.  The  bacteria  and  sus- 
pended matters  in  the  water  are  largely  retained  in 
the  filter-bed.  The  coagulant  may  also  reduce  the 
color  and  dissolved  organic  matter  in  the  water  to  a 
much  greater  extent  than  would  be  possible  with 
slow  sand-filters. 

Which  of  these  methods  of  purification  is  prefer- 
able, in  any  given  case,  must  be  determined  from  care- 
ful considerations  of  the  quality  and  character  of  the 
water,  the  results  desired  and  the  relative  costs  of  the 
processes  both  for  installation  and  operation. 


PURIFICATION  BY  SLOW  SAND-FILTRATION.   J? 
THEORY  OF  SLOW  SAND-FILTRATION. 

The  foreign  substances  carried  in  water  are  either 
mineral  or  organic,  and  they  are  dependent,  to  a  cer- 
tain degree,  on  each  other.  The  organic  matter  is 
found  first  as  living  organisms,  vegetable  or  animal, 
which  float  or  have  the  power  to  move  about  in  wa- 
ter; second,  as  the  products  of  organic  life,  such  as 
albumen,  urea  and  tissue,  which  may  be  dissolved  in 
the  water  or  suspended  in  it,  and  third,  as  products 
of  the  decomposition  of  organic  matter.  In  the  lat- 
ter class  belong  the  salts  of  ammonia  and  of  carbonic 
and  nitric  acids,  which  are  absorbed  by  growing  vege- 
tation as  food.  The  carbon  and  nitrogen  in  organic 
matter  are  constantly  changing  from  organic  to  min- 
eral matter  and  back  again.  The  organic  matter 
found  in  water  consists  mainly  of  carbon,  hydrogen, 
nitrogen  and  oxygen.  The  process  of  decomposition 
may  be  said,  in  a  general  way,  to  consist  of  first  the 
oxidation  of  the  carbon,  which  leaves  the  nitrogen 
combined  with  hydrogen  in  the  form  of  ammonia, 
and  subsequently  the  union  of  the  uncombined  oxy- 
gen with  the  ammonia,  converting  it  into  nitric  acid 
and  water.  This  series  of  changes  requires  the  pres- 
ence of  oxygen  and  of  some  earthy  or  alkaline  base 
in  the  water  with  which  the  acids  can  combine,  when 
formed.  Further,  the  presence  of  certain  micro-or- 
ganisms is  necessary  to  initiate  and  carry  the  process 
through  to  completion. 

In  surface  waters  there  are,  therefore,  constantly 
going  on  two  actions,  assisted  by  contact  with  the 


7§  WATER  FILTRATION  WORKS. 

air  and  the  action  of  the  sun's  light  and  heat.  These 
are  the  oxidation  of  the  elements  of  the  organic  mat- 
ter, and  their  absorption  by  the  various  forms  of  vege- 
tal and  animal  life.  This  process  only  goes  on  in  the 
presence  of  light.  In  pure  ground-waters  we  fail  to 
find  the  presence  of  nitrates,  but  in  ground-waters 
previously  polluted,  and  in  the  bottoms  of  deep  ponds, 
reservoirs  or  lakes  we  often  find,  due  to  the  absence 
of  uncombined  oxygen  and  to  the  absence  of  light, 
the  presence  of  free  ammonia  and  nitrites,  interme- 
diate products  of  the  regeneration  of  decaying  or- 
ganic matter.  The  plant-life  which  results  from  the 
absorption  of  the  oxidized  ammonia  is  called  in  chem- 
ical analyses,  the  albumenoid  ammonia. 

Shallow  stagnant  bodies  of  water,  which  in  the  heat 
of  summer  are  full  of  animal  and  vegetal  life,  become 
foul  in  time  because  decay  gets  ahead  of  growth,  and 
the  products  of  decomposition  accumulate. 

The  color  acquired  by  surface  waters,  apart  from 
turbidity,  is  derived  from  leaves,  grass,  peat,  etc.,  by 
long  contact.  It  contains  considerable  nitrogen,  and 
is  usually  very  stable  in  character.  For  the  removal 
of  this  matter  it  is  necessary  to  treat  the  water  with 
hydrate  of  aluminum,  which  combines  with  the  color- 
ing matter  and  gives  a  clear,  colorless  water  on  the 
precipitation  of  the  coagulant,  or  its  removal  by  rap- 
idly operated  filters. 

Pure  water  should  have  no  odor.  If  the  odor  is 
caused  by  dissolved  gases,  it  will  leave  when  the  wa- 
ter is  boiled.  If  it  comes  from  suspended  or  dissolved 
organic  matter,  it  may  vanish  when  the  water  is 


PURIFICATION  BV  SLOW  SAND-FILTRATION.  79 

boiled,  but  may  again  develop.  The  odors  from  sus- 
pended organic  matter  and  vegetation  may  some- 
times come  from,  the  decay  of  the  matter,  but  often 
they  are  caused  by  the  organisms  themselves.  Some- 
times the  removal  of  odors  is  a  difficult  matter,  re- 
quiring a  special  line  of  treatment. 

As  stated  before,  the  change  of  the  decaying  mat-' 
ter  from  organic  nitrogen  into  the  salts  of  nitric  acid 
can  only  be  brought  about  in  the  presence  of  bacteria. 
The  fact  had  long  been  known,  but  it  was  not  until 
about  1890  that  it  was  possible  to  isolate  the  nitrify- 
ing organism.  In  that  year,  by  the  independent  la- 
bors of  Edwin  O.  Jordan  and  Mr§.  Ellen  H.  Richards, 
of  the  Massachusetts  State  Board  of  Health,  Dr. 
Percy  F.  Frankland  and  Grace  Frankland  and  Wino- 
gradsky,  abroad,  the  organism  was  isolated;  since  that 
time  much  intelligent  investigation  has  been  under- 
taken to  determine  the  conditions  which  are  most  fa- 
vorable for  the  life,  propagation  and  activity  of  the 
organism. 

The  organism  is  present  and  active  in  the  presence 
of  oxygen,  in  all  normal  surface  waters  and  probably 
also  in  falling  rain.  Among  the  conditions  which  are 
essential  to  the  activity  of  the  organism  are  the  pres- 
ence of  oxygen,  organic  matter,  moisture  and  some 
alkali  and  a  temperature  suitable  to  foster  the  growth 
of  vegetation.  In  slowly  passing  water  containing 
these  necessary  ingredients  through  beds  of  sand, 
the  conditions  for  rapid  nitrification  are  gradually  es- 
tablished. The  nitrifying  organisms  in  the  applied 
water  become  attached  to  the  sand  grains,  mostly  in 


So  WATER  FILTRATION  WORKS. 

the  upper  layers  of  the  sand,  and  attack  the  organic 
matter  in  the  water,  which,  to  a  considerable  degree, 
appears  to  unite  with  certain  constituents  of  the  fil- 
tering materials.  The  organic  matter  is  resolved 
finally  into  soluble  mineral  salts,  which  pass  out  in  the 
effluent.  The  conditions  under  which  the  most  per- 
fect chemical  purification  takes  place  seem  to  be 
also  the  most  favorable  for  the  removal  of  the  bac- 
teria in  the  applied  water.  The  cause  of  the  death 
and  destruction  of  the  bacteria  may  lie  partly  in  the 
absorption  of  the  oxygen  of  the  applied  water  in  the 
process  of  nitrification,  but  probably  they  are  them- 
selves oxydized  the  same  as  other  organic  matter.  It 
cannot  be  said  they  die  from  lack  of  food-supply  or 
lack  of  oxygen,  for  there  is  generally  a  sufficient 
amount  of  ammonia  in  the  effluents  of  filters  to  sup- 
port a  considerable  bacterial  life;  and  it  is  known  that 
there  are  certain  species  of  bacteria  that  can  live  with- 
out the  presence  of  oxygen. 

Action  of  Slow  Sand-filters. — Recent  investigations 
have  demonstrated  that  in  slow  sand-filters  in  efficient 
service,  showing  a  normal  reduction  of  bacteria,  a  film 
of  gelatinous  material  forms  around  the  sand  grains 
whereby  most  of  the  bacteria  are  mechanically  re- 
tained under  conditions  that  are  not  favorable  for 
their  existence.  This  gelatinous  material  is  com- 
posed probably,  in  part,  of  dead  or  resting  bacteria. 

Efficiency. — In  discussing  the  results  obtained  by 
slow  sand^filtration  there  are  three  phases  which 
should  be  considered;  these  are:  bacterial  efficiency, 
bacterial  purification  and  hygienic  efficiency.  As  the 


PURIFICATION  BY  SLOW  SAND-FILTRATION.    8 1 

result  of  the  experience  of  many  years,  it  is  known 
that  the  number  of  bacteria  found  in  the  effluents  of 
slow  sand-filters  does  not  necessarily  represent  the 
number  which  have  passed  through  with  the  water, 
and  hence  bacteriological  analyses  of  filter-effluents 
may  not  be  a  correct  index  of  the  percentage  re- 
moval of  the  bacteria.  A  large  number,  and  in  some 
cases  all,  of  the  bacteria  found  in  the  effluent,  grow 
in  the  lower  part  of  the  filters  and  the  underdrains, 
and  there  is  as  yet  much  difficulty  in  distinguishing 
between  the  latter  and  those  which  pass  through  with 
the  water.  Plagge  *  holds  the  view  that  even  dis- 
ease germs  may  multiply  in  badly  managed  filters. 
The  bacterial  efficiency  is  the  percentage  which  the 
number  of  bacteria  found  in  the  effluent  water  is 
of  the  number  of  bacteria  in  the  raw  water.  The 
bacterial  purification  is  the  percentage  which  the 
bacteria  actually  removed  by  filtration  is  of  the  num- 
ber of  bacteria  in  the  water  applied,  and  is  consider- 
ably higher  than  the  bacterial  efficiency.  The  ex- 
periments with  special  growths  of  bacteria  at  the 
Lawrence  Experiment  Station,  in  1894  and  1895,  m~ 
dicate  that  the  normal  bacterial  purification  from 
slow  filtration  ranges  from  99  to  100  per  cent.  The 
hygienic  efficiency  is  regarded  as  the  percentage  re- 
moval by  filtration  of  the  bacteria  capable  of  produc- 
ing disease.  The  hygienic  efficiency  is  probably  fully 
as  great  as  the  bacterial  purification. 

*  Untersuchungen  uber  Wasserfilter.  Veroffentl.  aus  dem  Ge- 
biete  des  Militar-Sanitatswesens.  Med.  Abth.  des  koenigl. 
preuss.  Kriegsministerium.  Berlin,  1895. 


82  WATER  FILTRATION   WORKS. 

The  percentage  basis  of  expressing  bacterial  effi- 
ciency is  unfair  because  with  water  low  in  bacteria 
the  percentage  will  be  very  high,  but  with  polluted 
water  it  may  still  be  high  and  yet  allow  a  great  num- 
ber of  bacteria  to  appear  in  the  effluent.  The  German 
standard  of  100  per  c.c.  seems  to  be  based  on  a  more 
rational  idea,  but  this  is  also  open  to  the  objection  that 
it  is  not  universally  applicable.  If,  for  instance,  the 
water  were  sewage  polluted  there  might  be  many 
pathogenic  bacteria  in  this  100,  while  if  the  water 
were  not  sewage  polluted,  but  contained  several  hun- 
dred of  the  ordinary  water  bacteria,  it  would  be  per- 
fectly unobjectionable.  Such  results  must,  there- 
fore, be  interpreted  with  a  knowledge  of  the  charac- 
ter of  the  raw  water  as  to  sources  of  pollution. 

Influence  of  Character  of  Water. — The  influence  of 
the  character  of  the  water  upon  the  results  of  slow 
sand-filtration  is  very  decided.  The  available  infor- 
mation shows  clearly  that  there  are  some  waters  sat- 
urated with  oxygen  and  containing  small  amounts  of 
organic  matter,  which  may  be  successfully  purified  by 
continuous  filtration.  On  the  other  hand  water  con- 
taining very  little  or  no  free  or  dissolved  oxygen, 
and  large  amounts  of  organic  matter,  cannot  be  puri- 
fied successfully  by  the  continuous  method,  but  will 
require  an  intermittent  application  of  the  water  to  the 
filters  in  small  doses  whereby  a  sufficient  amount  of 
oxygen  is  carried  down  into  the  sand  to  effect  the 
requisite  oxidation. 

The  results  obtainable,  therefore,  in  the  filtration  of 
a  polluted  water  may  vary  with  seasonal  changes,  ac- 


PURIFICATION  BY  SLOW  SAND-FILTRATION.   83 

cording  to  the  amount  of  free  oxygen  in  the  water. 
When  the  oxygen  is  high  the  rate  of  filtration  may 
also  be  high.  The  amount  of  oxygen  present  in  the 
water  may  be  an  indication  of  whether  the  water 
should  be  filtered  intermittently  or  continuously. 
The  experience  at  the  Lawrence,  Mass.,  Experiment 
Station  has  shown  that  free  oxygen  is  never  absent 
from  the  effluents  of  slow  sand-filters  at  the  station, 
although,  at  times,  the  percentage  is  very  low,  par- 
ticularly in  the  filters  operated  continuously.  The 
fact  that  the  amount  of  free  oxygen  in  water  is  least 
in  summer  weather,  when  the  organisms  of  nitrifica- 
tion are  most  active,  is  significant.  In  Lawrence  the 
percentage  of  dissolved  oxygen  in  the  Merrimac 
River  and  in  the  effluents  of  the  water-filters  de- 
creases gradually  from  the  winter  months  to  the  mid- 
dle of  September,  and  then  gradually  increases  again 
as  the  winter  months  return.  It  is  also  noted  that  the 
effluents  from  the  intermittent  filters  contain,  almost 
uniformly,  a  higher  percentage  of  dissolved  oxygen 
than  the  effluents  from  the  continuous  filters.  The 
general  results  obtained  from  the  Lawrence  experi- 
ments indicate  that  in  the  treatment  of  the  Merrimac 
River  water  by  slow  sand-filtration  it  is  possible  to  re- 
move all  the  suspended  matter,  and  a  variable  amount 
of  the  color  and  dissolved  organic  matter,  and  that 
old  filters  are  as  efficient  as  new  in  removing  albu- 
menoid  ammonia.  The  experience  in  regard  to  the 
removal  of  suspended  matter  at  other  experiment 
stations  has  shown,  however,  that  slow  sand-filters 
may  fail  to  give  satisfactory  results  in  this  regard. 


84  WATER  FILTRATION   WORKS. 

Influence  of  the  Size  and  Character  of  the  Sand  on  the 
Efficiency  of  Slow  Sand-filtration. — The  physical  char- 
acteristics of  sand  may  be  determined  by  sifting  sev- 
eral samples  through  a  series  of  sieves  of  different 
meshes,  and  then  determining  the  percentage  by 
weight  of  each  size  and  the  relations  the  different 
samples  bear  to  each  other.  Using  the  nomenclature 
of  the  Massachusetts  State  Board  of  Health,*  the  Ef- 
fective Size  of  a  sand  is  the  size  of  the  grain  in  milli- 
metres, such  that  10  per  cent,  of  the  grains  in  the 
sample  is  finer  than  itself,  and  the  Uniformity  Co- 
efficient expresses  the  ratio  of  the  size  of  grain  such 
that  60  per  cent,  is  finer  than  itself  to  the  size  such 
that  10  per  cent,  is  finer  than  itself.  Since  the  purifi- 
cation in  the  filter  is  brought  about  by  the  passage  of 
the  water  between  the  sand  grains,  it  is  evident  that 
the  presence  of  large  stones  in  the  sand  will  not  add 
to  its  value;  the  smaller  the  percentage  of  particles  of 
larger  grain  than  those  of  the  effective  size,  the  less 
waste  material  there  will  be  in  the  filter,  and  the  more 
uniform  will  be  the  passage  of  the  water  through  it 
in  all  parts  of  the  beds,  both  as  to  velocity  and  as  to 
quantity. 

Influence  of  Compacting  of  Sand. — The  resistance  to 
the  motion  of  the  water  through  the  sand,  due  to  the 
compacting  of  the  surface  under  service,  gradually, 
to  a  slight  extent,  reduces  the  capacity  of  a  slow  sand- 
filter  plant.  This  is  caused  partly  by  the  settling  of 


*  For  methods  of  analyzing  sands,  see  article  by  Allen  Hazen, 
in  Report  of  Mass.  State  Board  of  Health,  1892. 


PURIFICATION  BY  SLOW  SAND-FILTRATION.   85 

the  sand  in  the  water  and  the  compacting  of  the  sur- 
face by  the  workmen  in  cleaning. 

Sand,  to  be  suitable  for  filter  purposes,  should  be 
free  from  clay  or  calcareous  materials,  as  these  have 
a  tendency  to  cement  the  sand  grains  together  and 
produce  other  disturbing  elements  tending  to  reduce 
the  efficiency,  both  by  increasing  the  frictional  resist- 
ances and  by  producing  sub-surface  clogging.  The 
grains  of  sand  should  also  be  as  uniform  in  size  as 
possible,  because  the  greater  the  variation  in  size,  for 
any  given  effective  size,  the  more  compactly  the  sand 
will  settle  under  the  action  of  the  flowing  water,  and 
the  greater  the  frictional  resistances  will  become. 
The  filling  of  the  filters  from  below,  as  well  as  the  es- 
cape of  air  upward  through  the  sand,  will  tend  to  re- 
adjust the  grains,  and  if  there  is  a  great  variation  in 
their  size,  they  can  pack  more  closely  than  if  they  are 
all  of  one  size. 

The  available  information  regarding  the  effect  of 
the  size  of  the  sand  grains  on  the  efficiency  of  a  given 
sand  in  removing  bacteria,  in  a  filter  which  has  been 
in  service  a  long  time,  shows  little  to  warrant  the  be- 
lief that  the  efficiency  depends  much  on  the  effective 
size,  within  certain  limits.  The  experience  at  the 
Lawrence  Experiment  Station  goes  to  show  that  the 
percentage  of  bacteria  which  will  pass  through  filters 
with  sands  of  effective  sizes  of  .14  to  .38  millimetres, 
which  have  been  long  in  service,  is  practically  inde- 
pendent of  the  effective  size.  (See  Fig.  3.)  The  num- 
ber of  bacteria  in  the  effluent  seems  to  depend  more 
on  the  number  in  the  applied  water  than  upon  any 


86 


WATER  FILTRATION    WORKS. 


other  factor.  Even  sands  with  an  effective  size  of  .48 
millimetres  show  as  high  bacterial  efficiency  as  the 
finer  sands,  but  require  a  longer  time  to  give  normal 
results.  Observations  on  filters  of  coarse  sands,  in 


PERCENT  AQ'E,  BACTERIAL  EFFICIENCY 

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RATE  OF  FILTRATION  IN  MILLION  GALLONS  PER  ACRE  PER  DAY 

FIGt  3.— EFFECT  OF  SIZE  OF  SAND  GRAINS  ON  EFFICIENCY  OF 
FILTRATION. 

operation  since  1889,  go  to  show  that  as  they  grow 
older  in  service,  they  resemble  more  and  more 
filters  of  fine  sand.  The  chief  points,  however,  in 
which  the  size  of  the  grains  has  the  most  influence  are 
that  filters  of  coarse  sand  require  a  longer  time,  after 
being  placed  in  service,  to  yield  effluents  of  normal 
bacterial  contents,  and  they  are  more  sensitive  to  dis- 
turbing influences  than  filters  of  fine  sand. 

Influence  of  Depth  of  Filtering  Materials. — The  influ- 
ence of  the  depth  of  the  filtering  material  on  the  effi- 
ciency of  filtration  is  felt  principally  in  the  steadying 


PURIFICATION  BY  SLOW  SAND-FILTRATION.  87 

action  afforded  by  deep  layers  on  the  velocity  of  flow 
of  the  water  through  the  sand,  and  by  the  bacterial 
action  which  takes  place  in  the  lower  part  of  deep  fil- 
ters. While  deep  filters  are  more  efficient  than  shal- 
low ones,  the  latter  are  fairly  satisfactory  under  fa- 
vorable conditions.  As  far  as  the  data  now  at 
hand  can  be  interpreted,  it  seems  that  a  depth  of  12 
inches  in  a  filter  long  in  effective  service,  will  give 
nearly  as  good  results  as  a  greater  depth,  provided 
there  is  no  outside  disturbing  influence;  but  if  such 
disturbance  should  occur,  its  effect  upon  the  effi- 
ciency of  nitration  will  be  more  marked  and  of  longer 
duration  than  in  the  case  of  deep  filters. 

Fig.  4,  which  shows  the  average  results  from  an- 
alyses of  the  materials  of  ten  filters  5  feet  deep  at  the 
Lawrence  Experiment  Station,  exhibits  the  accumu- 
lation of  organic  matter  and  bacteria  in  slow  sand- 
filters  in  successful  operation.  Four  of  these  filters 
were  intermittent  and  six  were  of  the  continuous 
type.  The  diagram  shows  that  the  greatest  amount 
of  the  work,  in  the  retention  of  the  bacteria  and 
stored  nitrogenous  matter,  is  being  clone  in  the  top  6 
or  8  inches  of  the  filters,  and  that  all  but  an  insignifi- 
cant proportion  of  the  bacteria  and  nitrogen  are  re- 
tained in  the  upper  inch.  As  the  depth  below  the  sur- 
face increases  the  number  of  bacteria  and  percentage 
of  stored  nitrogen  decrease  very  rapidly  until  a 
depth  of  about  i  foot  is  reached,  and  then  more 
slowly,  but  still  perceptibly  up  to  about  3  feet  or 
more.  The  stored  nitrogen  below  this  depth  may 
represent,  according  to  Mr.  Clark,  the  normal  quan- 


88 


WATER  FILTRATION    WORKS. 


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FIG.  4. — DIAGRAM  SHOWING  THE  RETENTION  OF  BACTERIA  AND 
STORED  NITROGEN  IN  TEN  SLOW  SAND-FILTERS  AT  THE  EX- 
PERIMENT STATION,  LAWRENCE,  MASS, 


PURIFICATION  BY  SLOW  SAND-FILTRATION.    89 

tity  which  existed  before  the  filters  were  placed  in 
operation.  This  nitrogenous  matter  in  the  filters  is 
the  film  of  gelatinous  matter  arranged  around  the 
grains  of  sand;  its  existence  in  the  lower  portions  of 
deeper  filters  in  part  explains  the  greater  and  more 
uniform  efficiency  of  deep  filters  in  removing  bacteria. 

Because  of  their  greater  sensibility  to  disturbing 
influences,  filters  of  a  depth  of  i  or  2  feet  would  not 
be  so  reliable  in  operation  as  deep  filters.  Experi- 
ments made  at  Lawrence  on  scraping  filters  2  feet 
deep  and  5  feet  deep  respectively,  showed  that  scrap- 
ing did  not  affect  the  number  of  bacteria  in  the 
effluent  of  the  filter  5  feet  deep,  while  in  the  shallow 
one  scraping  was  almost  invariably  followed  by  a 
large  increase  in  the  number  of  bacteria  passing 
through,  and  caused  Bacteria  Prodigiosis,  applied 
with  the  water,  to  appear  in  the  effluent  for  3  or  4 
days  following  the  scraping.  Numerous  other  records 
testify  to  the  greater  reliability  of  deep  filters,  and 
good  practice,  in  the  light  of  this  experience,  would 
require  that  filter-beds  be  made  from  4  to  5  feet 
deep,  according  to  circumstances. 

The  usual  European  practice  is  to  give  the  filter- 
ing materials  a  depth  of  from  2  to  4  feet,  and  to  al- 
low this  depth  to  be  diminished  by  repeated  scrapings, 
as  the  beds  clog,  to  from  i  to  2  feet.  When  the 
minimum  allowable  depth  has  been  reached,  the  sand 
taken  out  in  the  periodical  cleanings  is  replaced  and 
the  filter  brought  back  to  its  original  depth.  In  Ger- 
many the  Imperial  Board  of  Health  has  specified  that 
the  sand  should  never  be  reduced  to  less  than  12 


go  WATER  FILTRATION   WORKS. 

inches  in  depth  by  cleaning,  and,  when  possible,  a 
greater  depth  than  this  should  be  maintained. 

The  effect  of  the  uniformity  coefficient  and  the 
size  of  the  sand  have  already  been  discussed.  There 
is  another  qualification  necessary  for  effective  oper- 
ation, respecting  the  filtering  materials.  That  is,  the 
necessity  for  exercising  great  care  in  the  sorting  and 
placing  of  the  sand  in  the  filter.  Layers  of  fine  sand 
or  loam  in  the  body  of  the  filter  must  be  guarded 
against,  as  they  will  cause  sub-surface  clogging,  and 
therefore  reduced  efficiency.  Such  layers  have  been 
tried  in  Holland,  and  have  been  experimented  with 
at  Lawrence,  always  with  the  above  result.  It  is  ad- 
visable to  have  the  sand  as  nearly  uniform  as  possi- 
ble in  size  of  grain  from  top  to  bottom  of  the  filter- 
ing material,  and  to  so  place  the  sand  in  the  beds  that 
there  will  be  no  planes  of  lamination  or  layers  of  dif- 
ferent degrees  of  compactness. 

Effect  of  the  Loss  of  Head  Upon  the  Efficiency  of  Slow 
Sand-filters. — The  difference  of  level  between  the 
height  of  the  surface  of  the  water  on  the  filters  and 
the  height  to  which  the  water  would  rise  in  a  vertical 
pipe  attached  to  the  outlet  of  the  underdrains,  when 
the  filter  is  in  operation,  is  called  the  Loss  of  Head. 
This  loss  of  head  measures  the  resistance  offered  to 
the  passage  of  the  water  through  the  filter,  and  de- 
pends upon  the  quantity  of  water  filtered  per  unit  of 
time,  the  age  of  the  filter  and  other  causes.  It 
has  been  the  general  European  practice  to  limit  this 
loss  of  head  to  from  24  to  30  inches,  in  the  belief  that 
high  heads  compact  the  sand  and  also  cause  local 


PURIFICATION  ^Y  SLOW  SAtiD-FlLTRATlON.   9! 

breaking  of  the  fine  sediment  layer  on  the  surface, 
thus  permitting  water  to  pass  through  at  greatly  in- 
creased rates  with  resulting  reduced  efficiency. 

The  experience  of  recent  years,  however,  at  the 
Lawrence  Experiment  Station,  has  pointed  to  the 
conclusion  that  neither  of  these  circumstances  exer- 
cises an  important  influence  toward  deterioration  in 
the  bacterial  efficiency  in  properly  operated  filters, 
wrhen  the  loss  of  head  is  allowed  to  equal  the  com- 
bined depth  of  the  sand  and  its  covering  of  unfiltered 
water.  In  some  cases  abnormally  great  numbers  of 
bacteria  were  obtained  under  high  heads,  but  their 
presence  was  satisfactorily  explained  in  most  cases. 
Under  some  conditions,  however,  as  found  at  Cin- 
cinnati by  Mr.  Geo.  W.  Fuller,  a  head  greater  than 
the  depth  of  the  water  over  the  sand  was  found  to  be 
detrimental,  because  the  water  contained  a  large 
amount  of  air  in  solution  which  was  liberated  when 
the  head  became  negative;  and  this,  rising  in  the  form 
of  bubbles,  disturbed  the  filtering  materials  and 
brought  about  reduced  efficiency.  Generally  speak- 
ing, negative  heads  should  be  avoided  when  possi- 
ble, though  their  use  may  not  always  be  unsatisfac- 
tory. In  many  of  the  European  filters  the  loss  of 
head  is  limited  by  the  method  of  construction  of  the 
beds.  At  Berlin  the  limit  is  about  24  inches  and 
at  Hamburg  about  28  inches.  The  idea  is  com- 
monly held  abroad,  however,  that  not  only  are  high 
heads  dangerous,  but  that  after  a  limit  of  about  2 
feet  has  been  reached  the  head  will  increase  in  a 
very  much  more  rapid  proportion  than  the  quantity 


92  WATER  FILTRATION   WORKS. 

of  water  filtered  between  scrapings.  I  can  find  no 
careful  investigations  of  this  subject,  however,  ex- 
cepting those  of  the  Massachusetts  State  Board  of 
Health.  This  is  a  question  that  vitally  concerns  the 
cost  of  operation,  because  the  less  frequently  the  fil- 
ters require  cleaning,  the  less  will  be  the  cost  of  oper- 
ation. In  removing  the  film  of  sediment  on  the  sur- 
face of  the  filter  practical  considerations  make  it  im- 
possible to  remove  less  than  a  certain  depth  of  the 
material  at  one  time.  This  depth  is,  within  limits,  in- 
dependent of  the  quantity  of  dirt  that  has  accumu- 
lated, and  any  expedient  that  will  lengthen  the  period 
of  time  between  scrapings  will  result  in  a  correspond- 
ing reduction  of  the  quantity  of  sand  to  be  washed 
per  unit  of  water  filtered,  as  well  as  a  reduction  of 
area  of  filter  necessarily  out  of  use  during  cleaning. 
The  use  of  high  heads  will  therefore  allow  of  a 
greater  length  of  time  between  cleanings  than  low 
heads,  in  waters  which  can  be  successfully  treated  by 
slow  sand-filters.  The  experiments  with  the  Law- 
rence experimental  filters  seem  to  indicate  that  the 
quantity  of  Merrimac  River  water  which  can  be  fil- 
tered between  scrapings  is  almost  proportional  to  the 
maximum  loss  of  head  allowed,  and  to  the  quantity  of 
water  filtered  per  acre  per  day,  up  to  about  five  mill- 
ion gallons,  and  that  very  fine  sands  require  more  fre- 
quent scraping  than  medium  or  coarse  sands. 

The  great  difference  in  physical  characteristics 
and  chemical  constituents  of  waters  from  different 
sources,  and  at  different  times  of  the  year,  makes  it 
impossible  to  state  that  the  use  of  high  heads  is 


PURIFICATION  BY  SLOW  SAND-FlLTRATlON.   93 

always  advisable  from  the  point  of  efficient  and  eco- 
nomical operation.  The  indications,  however,  point 
to  the  advisability  fin  all  cases,  of  studying  carefully 
the  particular  water  in  question,  at  various  seasons 
of  the  year,  to  determine  what  effect  high  heads 
would  have  and  their  bearing  on  the  design  of  the 
works. 

Effect  of  tJte  Depth  of  Water. — There  are  few  avail- 
able data  on  the  effect  of  allowing  the  water  to  stand 
at  a  considerable  depth  over  the  sand  in  the  filters. 
Usually  economical  construction  establishes  the  limit. 
This  is  ordinarily  from  3  to  5  feet  in  the  European 
filters;  it  is  seldom  less  than  3  feet,  particularly  in 
open  filters  where  ice  is  apt  to  form,  and  seldom  more 
than  5  feet  except  when  the  thickness  of  the  sand 
layer  has  been  reduced  by  frequent  scraping.  It  is 
not  probable  that  very  much  greater  depths  would 
have  any  unfavorable  effect,  either  on  the  ability  of 
the  filter  to  pass  large  quantities  of  water,  at  reason- 
able rates,  or  on  its  bacterial  efficiency. 

Effect  of  the  Rate  of  Filtration  on  the  Bacterial  Effi- 
ciency.— The  European  engineers  generally  incline  to 
the  belief  that  low  rates  of  filtration  are  necessary  to 
high  efficiency.  Thus,  the  rate  allowed  at  Hamburg 
is  1.6  million  gallons  per  acre  per  day,  and  at  Berlin 
2.57  million  gallons.  Most  of  the  other  German 
works  keep  below  this  latter  limit,  while  in  the  Eng- 
lish practice  the  rate  is  generally  under  two  million 
gallons  per  acre  per  day.  If  it  is  possible  to  success- 
fully use  higher  rates  than  these  it  is  evident  that  a 
saving  may  be  made  in  the  area  of  the  niters,  and 


94  WATER  FILTRATION  WORKS. 

thus  in  the  cost  of  construction,  as  well  as  in  the,  oper- 
ating, interest  and  sinking-fund  charges.  We  have 
evidence,  in  some  cases,  that  rates  very  much  higher 
than  these  have  been  successful.  The  question  is  one 
that  must  be  specially  decided  for  each  locality.  Thus 
at  Zurich,  Switzerland,  the  water  is  often  compara- 
tively low  in  bacteria,  but  high  in  constituents  for  the 
formation  of  the  surface  film,  and  being  free  from  tur- 
bidity, allows  of  rates  at  times  exceeding  ten  million 
gallons  per  acre  per  day,  with  excellent  results.  At 
Lawrence  the  rates  have  occasionally  been,  with  old 
filters,  as  high  as  ten  million  gallons  per  acre  per  day 
in  successfully  filtering  the  Merrimac  River  water. 
Such  high  rates,  however,  are  not  recommended  for 
continuous  use. 

As  waters  vary  greatly  in  their  chemical,  bacterial 
and  physical  characteristics,  and  in  the  amount  and 
fineness  of  sediment  carried  in  suspension,  no  hard 
and  fast  rule  can  be  made  for  the  best  allowable  rate; 
this  can  only  be  approximated  in  advance  by  esti- 
mate and  finally  determined  by  actual  experience. 
Waters  which  are  normally  very  high  in  bacteria,  or 
in  organic  matter,  or  which  are  deficient  in  the  kind 
of  organic  matter  necessary  for  the  formation  of  the 
gelatinous  film  around  the  sand  grains,  or  which  con- 
tain a  considerable  amount  of  finely  comminuted 
clay  in  suspension,  will  require  lower  rates  than  wa- 
ters of  the  opposite  characters.  We  find  this  condi- 
tion prominently  recognized  in  the  European  works. 
The  Hamburg  rate  of  1.6  million  gallons  per  acre 
daily  for  the  black,-  muddy,  polluted  water  of  the  Elbe, 


PURIFICATION  BY  SLOW  SAND-FILTRATION.   $$ 

after  from  15  to  30  hours  of  settlement;  the  Berlin 
rate  of  2.57  for  the  ordinarily  clear  waters  of  the  Spree 
and  Havel,  and  the  Zurich  rate  of  about  7.5  million 
gallons  per  acre  per  day  for  the  perfectly  clear  lake- 
water,  are  probably  the  result  of  experience  with  sat- 
isfactory bacterial  purification  and  economy  of  oper- 
ation in  view.  With  the  water  of  the  Merrimac 
River,  at  the  Lawrence  Experiment  Station,  perfectly 
satisfactory  purification  has  been  attained  for  long 
periods  of  time  in  filters  of  considerable  age,  with 
rates  up  to  7  million  gallons  per  acre  per  day,  and 
in  some  cases  even  with  rates  reaching  10  million 
gallons. 

The  deleterious  effects  of  high  rates  will  be  felt  very 
much  more  in  filters  with  thin  than  with  thick  sand 
layers,  and  also  the  effects  will  be  more  noticeable  in 
a  new  filter  than  in  one  which  has  been  in  service 
many  months. 

Effects  of  Sudden  Changes  in  the  Rate  of  Filtration. — 
The  results  obtained  at  the  Lawrence  Experiment 
Station  indicate  that  sudden  changes  of  rate  should 
be  avoided,  as  they  are  likely  to  directly  affect  the 
bacterial  purification.  Generally  speaking,  an  in- 
crease of  rate  above  the  normal  at  which  the  filter  has 
been  operating  for  some  time  is  attended  by  a  marked 
increase  in  the  number  of  bacteria  in  the  effluent  for 
periods  of  from  several  hours  to  several  days,  and  this 
increase  in  number  usually  follows  the  change  of  rate 
in  about  such  a  time  as  to  suggest  that  the  multiplica- 
tion of  bacteria  in  the  effluent  is  due  largely  to  their 
detachment  from  the  sand  grains  near  the  surface  of 


96  WATER  FILTRATION 

the  filter.  A  comparatively  sudden  increase  of  rate 
from  below  the  normal  to  the  normal  rate,  as  in  in- 
termittent filters,  is  not,  as  a  rule,  in  filters  of  consid- 
erable age  followed  by  a  diminution  of  efficiency. 
Violent  changes  should  at  all  times  be  avoided  be- 
cause they  may  result  in  disturbing  mechanically  the 
filtering  materials,  and  consequently  directly  affect 
the  efficiency  of  the  process. 

Influence  of  the  Age  of  Slow  Filters  on  tJmr  Bacte- 
rial Efficiency. — When  a  new  filter  is  first  placed  in 
operation  it  does  not  at  once  begin  to  yield  pure  wa- 
ter. It  generally  requires  from  one  to  two  months  to 
establish  its  proper  biological  construction  and  give 
an  effluent  containing  a  low  number  of  bacteria.  This 
biological  construction  consists  principally  in  the  ac- 
cumulation of  organic  and  mineral  matter,  in  a  gelat- 
inous film,  around  the  sand  grains,  and  in  the  de- 
velopment of  the  nitrifying  organisms  by  which  the 
organic  matter  and  the  bacteria  are  retained  and  de- 
stroyed. This  power, of  retaining  and  destroying  the 
bacteria  in  the  applied  water  increases  with  the 
length  of  time  the  filter  has  been  in  operation.  This 
increased  bacterial  efficiency,  caused  by  greater 
length  of  service,  is  much  more  apparent  in  filters 
of  coarse  sand  than  in  those  constructed  with  fine 
sand,  and,  indeed,  as  filters  of  coarse  sand  increase 
in  age,  they  resemble,  both  in  bacterial  efficiency  and 
in  ability  to  pass  given  quantities  of  water,  filters  of 
fine  sand.  This  is  probably  due,  on  the  authority  of 
the  Massachusetts  State  Board  of  Health,  to  the 
more  closely  compacted  condition  of  the  sand,  caused 


PURIFICATION  BY  SLOW  SAND-FILTRATION.    9/ 

by  a  readjustment  of  the  sand  grains  in  refilling  the 
filters  from  below,  by  the  washing  in  of  fine  sedi- 
ment, and  the  retention  of  masses  of  organic  and  min- 
eral matter  on  the  sand  grains,  which  in  reality  re- 
duce the  effective  size  of  the  sand. 

Influence  of  Scraping  on  the  Bacterial  Efficiency  of 
Slow  Sand-filters. — The  theory  had,  until  recently, 
been  held  in  Europe  that  the  effectiveness  of  the  op- 
eration of  slow  sand-filters  depended  upon  the  forma- 
tion of  a  layer  of  sediment  upon  the  surface,  by  which 
the  bacteria  were  retained.  This  theory  was  formu- 
lated upon  the  studies  of  the  Berlin  filters  in  1887  by 
Piefke,  Pflugge  and  Proskauer,  and  was  quite  gener- 
ally endorsed  by  the  majority  of  writers  on  the  sub- 
ject. At  the  present  time,  however,  it  may  be  said 
that  most  of  the  prominent  engineers  of  Europe  look 
upon  bacterial  action  as  the  principal  factor.  Piefke,* 
of  Berlin,  contends  that  clay  particles  play  an  im- 
portant part  in  the  formation  of  the  surface  film,  as- 
serting that  as  the  result  of  experiments  he  found 
such  a  film  to  be  more  efficacious  than  a  film  com- 
posed largely  of  bacteria  and  algae.  The  most 
weighty  proof  that  such  a  film  is  not  indispensable  is 
advanced  by  the  Massachusetts  State  Board  of 
Health  in  the  four  following  propositions: 

i.  This  film  is  not  necessary  in  intermittent  filters, 
which  yield  as  high  results,  apparently,  when  this 
layer  is  cracked  and  peeled  off  by  the  action  of  the 
direct  rays  of  the  sun. 

*  Aphorism  iiber  Wasserversorgung  vom  hygienisch  technischen 
Standpunkt  ausbeobachted,     Zeitschrift  fiir  Hygiene,  1889. 


9  WATER  FILTRATION    WORKS. 

2.  In  the  studies  of  continuous  niters  of  fine  or 
medium  sand  it  has  been  observed  that  in  more  than 
100  instances  it  was  possible  to  remove  from  .10  to 
.30  inch  in  depth  of  the  upper  layer  of  the  filter  with- 
out causing  a  diminution  of  efficiency. 

3.  It   has   been   observed   that   certain   filters   of 
coarse  sand  did  not  give  normal  bacterial  results  dur- 
ing the  first  months  of  their  operation,  even  when 
the  surface  coating  was  thick  enough  to  completely 
clog  the  filters,  and  yet  after  longer  service  their 
efficiency  increased  to  the  normal. 

4.  Chemical  analyses  of  the  sand  taken  from  filters 
at  different  depths  below  the  surface  showed  an  accu- 
mulation of  organic  matter,  it  being,  in  some  cases, 
50  per  cent,  of  that  at  the  clogged  surface  at  the 
depth  of  3  inches. 

Reinisch  has  also  stated,  from  his  studies  of  the 
Altona  filters,  that  too  much  significance  has  here- 
tofore been  given  to  the  surface  coating. 

The  studies  made  at  the  Lawrence  Experiment 
Station  indicate  quite  decisively  that  the  removal  of 
the  surface  layer  to  the  depth  of  an  inch  has  but  a 
very  slight  influence  upon  the  bacterial  efficiency,  in 
the  filtration  of  the  Merrimac  water,  and  that  the 
effects  of  such  deep  scraping  may  often  be  disguised 
by  other  considerations.  With  depths  of  more  than 
an  inch  the  effect  upon  the  bacterial  contents  of  the 
effluent  at  Lawrence  was  generally  very  marked. 

Raking  over  the  surface  to  the  depth  of  an  inch, 
as  compared  with  scraping,  has  not  shown,  under  the 
conditions  prevalent  at  Lawrence,  equally  good  re- 


PURIFICATION  BY  SLOW  SAND-FILTRATION.    99 

suits.  A  disturbance  of  the  sand  to  greater  depths 
than  this  invariably  results  in  reduced  efficiency  and 
long  delays  in  the  re-establishment  of  normal  ac- 
tion. The  ill-effects  of  scraping  were  more  ap- 
parent in  shallow  filters  than  in  those  having  deep 
sand  layers.  This  fact  suggests  that  the  steadying 
effect  of  deep  filters  is  a  great  safeguard  when  the 
plant  is  to  be  operated  by  unskilled  labor,  especially 
during  the  winter,  when  ice  is  apt  to  form,  and  when 
shallow  filters  would  require  the  most  intelligent  and 
careful  manipulation  to  yield  satisfactory  results. 

The  most  satisfactory  method  of  cleaning  slow 
filters,  as  evolved  both  from  American  and  European 
experience,  is  to  scrape  off  the  top  surface  to  a  depth 
of  about  one  half  to  three  quarters  of  an  inch  and 
then  rake  it  over  carefully  and  lightly  to  remove  the 
marks  of  the  boots  of  the  workmen.  ,This  process  is 
repeated,  when  necessary,  until  the  sand  layer  is 
reduced  to  the  minimum  thickness  allowed.  The 
refilling  with  washed  sand  immediately  after  each 
scraping  does  not  yield  satisfactory  results,  as  it  gen- 
erally produces  sub-surface  clogging  at  the  junction 
of  the  new  and  old  sand. 

Effect  of  the  Method  of  Application  of  Water  to  Inter- 
mittent Filters. — To  obtain  high  efficiency  in  inter- 
mittent filters  the  water  must  be  applied  in  such  a 
manner  as  to  avoid  disturbing  the  surface  of  the  sand 
layer.  When  the  water  is  flooded  over  the  top  of 
the  filter  the  air  held  in  the  body  of  the  sand  is  forced 
to  escape,  and  if  its  only  outlet  is  through  the  surface 
there  results  a  breaking  of  the  continuity  of  the  fil- 


IOO  WATER   FILTRATION    WORKS. 

ter  and  reduced  efficiency.  If,  however,  the  air  is 
forced  downward,  and  out  through  the  underdrains, 
these  ill-effects  are  very  largely  obviated. 

Effect  on  Bacterial  Efficiency  of  Method  of  Putting 
Slow  Sand-filters  in  Use  after  Scraping. — The  usual 
practice  of  filling  filters  after  scraping  has  been  to 
allow  filtered  water  to  slowly  flow  back  into  the 
underdrain  of  the  filter  and  gradually  rise  above  the 
surface  of  the  sand.  In  some  of  the  European  filters 
it  has  been  the  custom  to  waste  the  first  water  pass- 
ing through  after  a  scraping,  varying  the  quantity 
wasted  according  to  circumstances.  It  has  been 
found,  however,  that  in  nearly  all  cases  satisfactory 
results  can  be  obtained  by  filling  from  below,  allow- 
ing the  water  to  stand  a  short  while  before  placing 
the  filter  in  operation,  and  then  starting  filtration  at 
a  rate  below  normal.  In  this  manner  it  is  found  that 
a  sufficiently  good  effluent  can  be  obtained,  in  many 
cases,  without  wasting.  With  waters  low  in  the  ma- 
terials necessary  for  the  production  of  the  gelatinous 
film  on  the  sand  grains,  less  favorable  results  are  ob- 
tainable by  this  mejthod  of  starting;  in  such  cases 
wasting  may  be  necessary. 

Effect  of  Temperature  on  the  Efficiency  of  Slozv  Sand- 
filtration. — Observations  at  many  filtration-works 
indicate  that  the  reductions  of  bacterial  efficiency 
which  have  been  noted  in  extremely  cold  weather  have 
been  due  to  causes  which  could  be  removed  by  struc- 
tural and  operative  changes;  the  low  temperature 
of  the  water  not  being  chargeable  with  the  reduction 
of  efficiency.  Open  filters  which  have  shown  low 


PURIFICATION  BY  SLOW  SAND-FILTXATION.lOl 

efficiency  in  cold  weather  have,  upon  their  being  cov- 
ered over  to  protect  them  from  the  formation  of  ice, 
shown  again  their  normal  power  of  removing  bac- 
teria. This  has  happened  at  Zurich,  Berlin,  and  Koe- 
nigsberg.  The  filters  at  Altona  and  Hamburg  and  all 
the  cities  of  England  and  Holland  are  open,  and  but 
little  trouble  has  been  experienced  at  these  plants 
during  winter  weather.  Where  the  winter  tempera- 
ture is  such  that  many  days  of  severe  cold  may  follow 
in  succession,  producing  several  inches,  or  feet,  of  ice, 
it  will  generally  be  economical  to  cover  the  filters. 
This  subject  is  discussed  more  fully  on  page  120. 
The  reduction  of  efficiency  in  the  winter  months 
may  be  due  to  the  disturbance  of  the  top  surface  of 
the  sand  during  the  removal  of  ice;  to  the  freezing 
of  the  surface  after  scraping;  or  to  the  necessity 
of  compelling  certain  portions  of  the  filters  to  be 
cleaned  more  frequently  than  the  remainder  of  the 
area,  resulting  also,  perhaps,  in  abnormally  high  rates 
of  filtration  on  the  parts  so  cleaned. 

Conclusions. — From  a  careful  consideration  of  the 
observed  facts  it  is  seen  that,  under  favorable  condi- 
tions, the  process  of  slow  sand-filtration  may  be  very 
efficient  for  the  treatment  of  polluted  waters.  It  is 
also  seen  that  some  waters  cannot  be  successfully 
treated  by  this  process.  The  process  is  not  efficient 
for  the  removal  of  coloring  matter  dissolved  from 
leaves,  roots  and  grass,  peat  and  decaying  organic 
matter.  It  is  not  efficient  for  the  removal  of  turbid- 
ity caused  by  clay  in  a  very  finely  comminuted 
condition;  it  is  not  efficient  in  improving  the  chemi- 


IO2  WATER  FILTRATION   WORKS. 

cal  quality  of  the  water;  and  it  is  not  efficient  in  the 
treatment  of  waters  deficient  in  the  organic  matters 
necessary  for  the  formation  of  the  gelatinous  film 
around  the  grains  of  sand.  Further,  continuous  slow 
sand-filtration  is  not  capable  of  purifying  a  water 
highly  polluted  with  sewage  and  at  the  same  time  low 
in  dissolved  oxygen.  For  such  waters  intermittent 
filtration,  or  double  filtration,  may  be  necessary.  In 
point  of  efficiency  in  the  removal  of  bacteria  from  pol- 
luted waters,  under  proper  conditions,  howeyer,  this 
method  of  filtration  takes  first  rank  for  reliability 
over  all  other  practicable  processes  known  to-day. 
It  has  passed  the  experimental  stage,  as  a  process, 
and  is  known,  when  properly  applied  under  suitable, 
conditions,  to  be  safe,  satisfactory  and  economical. 


CHAPTER  IV. 

DESIGN,    CONSTRUCTION  AND  OPERATION  OF 
SLOW  SAND-FILTERS. 

DESIGNING. 

Per  Capita  Water  Consumption. — The  number  of 
filter-beds  required  to  supply  filtered  water  to  a  given 
population,  and  the  size  of  each  bed,  depend  princi- 
pally upon  the  per  capita  daily  water  consumption, 
and  upon  the  character  of  the  raw  water. 

The  per  capita  daily  water  consumption  of  the 
cities  of  the  United  States  is  generally  higher  in  large 
than  in  small  cities.  This  fact  is  one  of  the  elements 
which  makes  the  filtration  of  our  large  public  supplies 
a  matter  of  considerable  expense,  often  influencing  a 
city  to  defer  improvements,  in  the  hope  that  it  may 
become,  through  more  favorable  conditions,  better 
able  to  meet  the  expenditure  at  some  time  in  the 
future.  In  filtration-works  the  annual  cost  of  opera- 
tion and  the  original  outlay  for  construction  are  gov- 
erned by  this  item,  and,  therefore,  the  necessity  for 
avoiding  needless  waste  is  apparent  when  the  purifi- 
cation of  the  water  is  contemplated.  If  a  city  of 
100,000  people  uses  15,000,000  gallons  daily,  requir- 
ing a  filter-plant  costing,  say  $450,000  for  construc- 

103 


IO4  WATER  FILTRATION    WORKS. 

tion  and  about  $43,000  per  year  for  operation,  could 
get  along  with  10,000,000  gallons  per  day,  the  works, 
at  the  same  rate  as  above,  could  be  built  for  $300,000 
and  could  be  operated  for  $29,000  a  year.  The  dif- 
ference is  apparent.  It  is,  however,  the  duty  of  the 
engineer  to  solve  problems  on  a  business  basis  rather 
than  from  a  strictly  theoretical  point  of  view,  and  the 
question  of  waste  restriction  is  one  of  the  problems 
in  which  business  enters  to  a  very  large  extent. 
While  no  one  will  dispute  the  advantages  of 
economy,  public  economists  differ  radically  in  the 
means  proposed  for  bringing  about  their  ends.  In 
large  cities  with  long-established  customs,  with  pecu- 
liar industries,  with  special  necessities  for  the  use  of 
water  and  special  reasons  why  .a  large  amount  is 
wasted,  reforms  can  only  be  made  gradually,  and, 
as  it  were,  at  the  wish  of  the  people.  If  a  commission 
should,  in  such  a  city,  order  the  immediate  stoppage 
of  all  waste  and  insist  upon  the  placing  of  meters  on 
every  consumer's  supply-pipe,  urging  that  nothing 
could  be  done  in  the  way  of  purification  of  the  water 
until  such  measures  had  been  carried  out,  it  would 
fail  entirely  in  its  mission,  either  as  to  reducing  the 
waste  or  catering  to  the  public  interests  by  improv- 
ing the  water.  While  much  can  be  done  in  the  re- 
striction of  waste  in  cities,  if  the  question  is  properly 
approached,  it  cannot  be  done  in  a  day,  and  the  diffi- 
culties of  the  task  will  increase  with  the  magnitude 
of  the  city.  The  records  of  cities  using  meters  show 
almost  conclusively  what  can  be  done  in  this  direc- 
tion; but  when  it  comes  to  inaugurating  the  introduc- 


DESIGNING   SLOW  SANP-FILTERS.  iO$ 

tion  of  these  devices  in  large  cities,  where  such  action 
will  affect  realty  investments,  change  the  returns  on 
productive  property,  and  necessitate  the  expenditure 
of  large  sums  of  money  for  repairs  and  improvements 
to  plumbing,  there  is  sure  to  spring  up  opposition 
which  can  be  overcome  only  with  difficulty. 

It  is,  therefore,  better  to  look  the  matter  squarely 
in  the  face.  The  most  practicable  policy  is  to  propor- 
tion the  works  to  suit  the  actual  water  consumption 
at  the  time.  This  will  provide  all  the  water  the  peo- 
ple have  become  accustomed  to,  and  will  avoid  the 
semblance  of  a  water  famine  which  would  ensue  if  the 
reduction  of  the  supply  were  suddenly  brought  about. 
Then,  after  the  works  are  built,  is  the  time  to  begin 
lessons  in  waste  reduction.  By  first  metering  willing 
consumers,  of  whom  there  are  always  a  great  many  in 
large  cities,  the  doubtful  become  convinced  of  the 
benefits  of  the  system,  and  finally  enough  meter- 
takers  can  be  secured  to  force  into  line  those  who  op- 
pose meters  from  ulterior  motives.  By  such  a  pro- 
cedure the  consumption  can  be  gradually  cut  down, 
and  thus,  as  the  city  grows,  the  reduction  in  per 
capita  consumption  will  permit  the  original  works  to 
serve,  perhaps,  for  many  years  before  extensions  be- 
come necessary.  This  policy  is  not  wasteful  of  public 
funds,  and  is  possible  of  enforcement  in  many  cases. 
The  attempt  to  cram  meters  down  the  throats  of  an 
unwilling  public,  willy-nilly,  is  generally  productive  of 
a  species  of  mal-de-meter,  so  to  speak,  that  becomes 
endemic  and  difficult  to  eradicate;  the  prevailing 
symptoms  being  a  feverish  excitement  in  councils,  a 


106  WATER  FILTRATION   WORKS. 

chilly  reception  of  the  measure  by  the  press,  followed 
by  a  feeling  of  intense  depression  on  the  part  of  the 
friends  of  the  meter. 

The  small  per  capita  water  consumption  of  some  of 
the  large  European  cities  is  often  quoted  as  proof 
that  our  cities  are  extravagantly  wasteful  of  water, 
but  to  any  one  who  has  spent  considerable  time  in 
these  cities,  not  in  the  fine  hotels  where  rich  Ameri- 
cans congregate  to  be  fleeced,  but  in  the  homes 
of  the  middle  classes  and  in  the  smaller  hotels, 
the  reason  for  the  small  consumption  will  be  ap- 
parent on  starting  a  search  for  a  bath-tub  or 
water-closet.  They  do  not  waste  water;  they  do 
not  use  enough  of  it,  from  the  American  point 
of  view.  Manchester,  a  few  years  ago,  was  one 
of  the  favorite  cases  held  up  to  wasteful  Ameri- 
can cities  as  an  example  of  what  could  be  done 
in  the  matter  of  getting  along  without  water; 
she  is  no  longer  useful  for  that  purpose,  because 
since  the  building  of  the  sewers  and  the  intro- 
duction of  water-closets,  wash-stands  and  stationary 
tubs,  and  numerous  other  conveniences  that  are  to 
be  found  in  every  American  hamlet  of  3,000  people, 
the  consumption  is  gradually  climbing  up  to  where 
it  ought  to  be,  judging  from  the  American  stand- 
point. It  is  neither  practicable  nor  desirable  to  at- 
tempt to  limit  the  use  of  water  in  our  large  cities 
to  such  low  figures  as  are  quoted  for  some  of  the 
foreign  cities,  as  we  have  different  conditions  of 
national  temperament  and  municipal  and  govern- 
mental administration.  A  great  deal  can  be  done, 


DESIGNING  SLOW  SAND-FILTERS.  tO? 

however,  in  the  detection  of  useless  waste,  by  in- 
spection, or  other  means,  and  offenders  should  be 
brought  in  line,  so  as  to  keep  the  consumption  down 
to  the  lowest  practicable  limit,  in  order  to  save  ex- 
pense in  construction  and  in  the  operation  of  the 
works. 

Number  of  Filter-beds  Required,  and  Excess  Area 
to  Be  Provided. — Having  decided  upon  the  per  capita 
water  consumption,  the  most  advisable  rate  of  fil- 
tration, in  consideration  of  the  character  of  the 
water  and  the  sand,  the  proportioning  of  the  number 
of  beds  and  the  size  of  each  depends  upon  the 
amount  of  area  that  must  be  provided  in  excess,  to 
permit  of  the  periodical  cleaning  of  the  beds.  This 
excess  area  varies  greatly  in  the  extant  works,  rang- 
ing from  5  per  cent,  to  about  20  per  cent,  of  the 
total  area,  and  in  some  of  the  smaller  ones  being 
100  per  cent.  It  is  not  necessary,  usually,  to  propor- 
tion the  works  for  a  much  larger  population  than 
is  resident  in  the  city  when  they  are  completed, 
because  the  plant  will  be  capable  of  extension  at  a 
cost  probably  not  much  higher  in  rate  than  the  cost 
of  the  original  works,  with  the  possibility  of  defer^ 
ring  extensions  if  it  is  feasible  to  reduce  the  waste 
in  the  city. 

For  waters  carrying  a  good  deal  of  suspended 
matter,  or  particularly  rich  in  algae  growth,  the  re- 
quired proportion  of  excess  area  will  be  greater  than 
for  clear  waters,  because  in  the  former  cases  the  beds 
will  require  frequent  cleaning;  under  such  condi- 
tions, with  proper  preliminary  treatment  by  sedi- 


108  WATER  FILTRATION- 

mentation  or  other  methods  of  clarification,  it  is 
seldom  that  the  beds  will  require  cleaning  oftener 
than  once  a  week,  when  operating  at  ordinary  rates; 
while  with  clear  waters  it  is  seldom  that  the  beds 
will  require  cleaning  as  often  as  once  in  two  weeks. 
In  most  of  the  existing  works  the  average  period 
between  cleanings  is  about  a  month. 

After  deciding  upon  the  allowable  maximum  rate 
of  filtration,  the  proper  size  and  number  of  beds  may 
be  determined  when  the  maximum  daily  draft  on  the 
filters  is  known.  The  water  consumption  will  fluctu- 
ate with  the  time  of  day,  with  the  days  of  the  week, 
and  the  seasons  of  the  year.  The  maximum  to  be 
expected  should  not  exceed  the  average  daily 
draft  by  more  than  from  50  to  60  per  cent.,  and, 
therefore,  unless  there  are  storage  reservoirs  of  am- 
ple capacity  in  the  distribution  system,  the  beds 
should  be  proportioned  to  deliver  in  24  hours  1.5  to 
1.6  times  the  average  daily  draft,  in  order  that  the 
maximum  rate  of  filtration  may  not  be  exceeded.  A 
reservoir  sufficiently  large  to  balance  the  hourly  fluc- 
tuations in  draft  should  also  be  provided.  A  discus- 
sion of  the  proper  amount  of  storage  to  meet  this 
requirement  will  be  found  in  chapter  VIII. 

If  the  distribution  or  storage  reservoirs  are  large 
enough  to  balance  the  daily  fluctuations  of  draft,  the 
beds  may,  of  course,  be  designed  for  average  draft 
instead  of  maximum. 

As  has  already  been  discussed  on  pp.  93  to  96,  the 
increase,  within  reasonable  determinate  limits,  of  the 
rate  of  filtration  of  slow  sand-filters  operating  nor- 


DESIGNING  SLOW  SAND-FlLT£R$. 

mally  at  fairly  slow  rates  may  occasionally  be  per- 
missible for  short  periods  of  time,  depending  upon 
the  relative  pollution  of  the  water,  and  other  factors. 
Advantage  may  sometimes  be  taken  of  this  to  design 
the  filters  for  the  average  draft  of  water,  providing  a 
small  filtered-water  reservoir  to  balance  the  sudden 
changes  in  rate  of  draft,  and  depending  upon  the 
flexibility  of  the  filters  to  meet  the  daily  or  seasonal 
variations. 

The  total  necessary  effective  area  for  the  filter- 
beds,  including  the  surplus  area,  to  be  provided  to 
permit  of  the  periodical  cleaning  of  the  filters  as  they 
become  clogged  and  still  not  work  the  remaining 
beds  beyond  the  prescribed  limits,  may  be  found  from 
the  following  formula: 


A  =  total  necessary  area,  including  reserve,  in  acres. 
Q  =  total  quantity  of  water  to  be  filtered  daily  in 

million  gallons. 
r  =  rate  of  filtration  in  million  gallons  per  acre  per 

day. 
n  =  number  of  filter-beds,  including  reserve  beds. 


*  The  expression  — — —  will  generally  be  fractional,  but  in  the 

formula  use  the  -nearest  integer  as  follows  :  If  the  expression 
equals  or  is  less  than  i,  2,  3,  4,  etc.,  take  as  its  value  in  the 
formula  o,  i,  2,  3  etc.,  respectively.  If  it  is  greater  than  i,  2,  3, 
4,  etc.,  take  as  its  value  i,  2,  3,  4,  etc.,  respectively. 


H6  WATER  FILTRATION  WORKS. 

p= ordinary  minimum  number  of  days  of  service 

between  cleaning. 

c — number  of  days  eac'h  filter  is  out  of  service  While 
draining,  cleaning,  and  refilling. 

The  following  illustrations  will  serve  to  show  the 
relative  effects  on  the  size  of  the  beds,  of  different 
assumptions  regarding  the  operation  of  the  works, 
and  will  point  out  the  economies  which  may  be  ef- 
fected in  designing  and  operating  a  plant. 

Suppose  a  city  uses  100,000,000  gallons  of  water 
daily,  and  the  filters  are  to  operate  at  the  maxi- 
mum rate  of  5,000,000  gallons  per  acre  daily. 
The  plant  consists  of  20  beds,  the  average  period  be- 
tween cleanings  is  six  days,  and  each  filter  is  out  of 
service  three  days  for  cleaning,  resting,  and  refilling. 

1.  The  area  of  each  bed  would  be  1.5461  acres, 
requiring  a  total  area  of  30.92  acres. 

2.  If  the  lapse  of  time  between  cleanings  were 
thirty  days  the  area  of  each  bed  would  be  i.m  acres, 
and  the  total  area  22.22  acres. 

3.  If  in  the  first  instance  the  beds  were  out  of  ser- 
vice only  two  days  instead  of  three,  the  area  of  each 
would  be  1.33  acre,  and  the  total  area  26.6  acres. 

In  the  second  case  the  beds  were  designed  for 
monthly  cleanings,  and  2  beds  would  be  in  clean- 
ing, while  18  would  be  in  service.  If  now  a  period 
of  bad  water  were  to  come  on,  and  the  beds  required 
cleaning  every  six  days,  it  would  be  necessary  to 
have  7  beds  in  cleaning,  and  13  beds,  with  an  area 
of  14.55  acres  only,  would  be  in  service.  For  these 
13  beds  to  deliver  the  requisite  100,000,000  gallons 


DESIGNING   SLOW  SAND-FILTERS.  ill 

daily  the  rate  of  filtration  would  have  to  be  increased 
to  about  6.92  million  gallons  per  acre  per  day,  or  to 
a  rate  about  38  per  cent,  above  the  normal  rate. 
The  effect  of  changing  the  rate  by  this  amount 
might  not  be  so  dangerous  as  to  preclude  its  occa- 
sional occurrence  if  special  precautions  were  taken 
during  these  periods  to  insure  as  great  efficiency  as 
possible.  It  would,  therefore,  under  the  conditions 
assumed,  not  be  economical  to  proportion  the  beds 
upon  the  basis  of  weekly  cleanings,  as  that  assump- 
tion would  necessarily  increase  the  cost  of  the  plant 
by  about  60  per  cent.  On  the  other  hand,  no 
economy  would  result,  in  this  case,  in  designing  the 
beds  for  a  period  of  as  long  as  forty-five  days  between 
scrapings,  because  there  would  still  be  2  beds  in 
cleaning,  and  the  proportion  of  reserve  area  would 
be  the  same,  unless  the  number  of  beds  were  less 
than  17. 

Now  as  to  the  effect  of  changing  the  number  of 
beds,  still  assuming  the  same  quantities  for  con- 
sumption, rate  of  filtration,  a  thirty-day  period  be- 
tween scrapings  and  three-day  periods  of  rest: 

Supposing  ii  beds  were  built;  the  area  of  each 
would  be  2  acres,  and  the  total  area  22  acres,  thus 
requiring  0.22  acre  less  than  if  20  beds  'had  been 
built.  Assuming  that  the  cost  of  filtering  materials, 
roofing  and  flooring  are  the  same  per  square  foot  of 
area  for  filters  of  all  sizes,  an  assumption  not  far  from 
the  truth,  there  would  be  a  saving,  in  using  1 1  beds, 
of  9  division  walls  between  filters,  9  inlet  wells  with 
regulating  apparatus,  9  outlet  wells,  0.22  acre  of  fil- 


112  WATER  FILTRATION    WORKS. 

tering  materials,  roofing  and  flooring,  and  a  small 
saving  in  the  cost  of  underdrainage. 

The  increased  cost  and  disadvantages,  from  the 
use  of  ii  beds,  would  result  from  a  slightly  greater 
inconvenience  in  handling  the  sand  in  scraping  and 
refilling;  and,  in  open  filters,  where  ice  of  consider- 
able thickness  is  apt  to  form,  additional  difficulties 
in  scraping,  and  in  disposing  of  the  ice. 

It  is  evident,  however,  that  economy  of  construc- 
tion favors  large  beds.  In  any  case  it  is  necessary 
to  decide,  first,  the  maximum  rate  of  filtration  to  be 
allowed  and  then  to  determine  the  corresponding 
number  of  beds,  regard  being  had  to  the  period  be- 
tween scrapings  that  will  make  the  total  area  the 
least  while  insuring  that  the  allowable  maximum 
rate  on  the  beds  in  use  will  not  be  much  exceeded  if 
the  period  between  scrapings  is  occasionally  re- 
duced to  six  days.  The  maximum  practicable  size 
for  filter-beds  has  not  been  definitely  determined. 
The  largest  in  use  are  the  uncovered  beds  at  Ham- 
burg, which  have  an  area  of  1.88  acre  each,  and 
have  given  satisfactory  results.  Most  of  the  beds  of 
the  other  European  filters  are  from  .5  to  1.5  acres  in 
area  each.  Frequently  local  prices  of  land,  of  labor 
and  of  materials  may  have  an  important  influence 
in  deciding  the  size  of  the  beds.  It  would  hardly  be 
necessary  in  any  case  to  make  the  beds  larger  in 
area  than  2  acres  each,  as  even  in  very  large  plants 
no  great  economy  would  result  from  using  larger 
sizes. 

In  the  examples  which  have  just  been  discussed, 


DESIGNING   SLOW  SAND-FILTERS.  113 

if  the  maximum  rate  had  been  fixed  at  7.5  million 
gallons  per  acre  daily,  it  would  have  been  found 
more  economical  to  use  16  beds  and  a  period 
of  service  of  forty-five  days  between  scrapings. 
In  this  case  the  15  beds  in  service  would  ordi- 
narily deliver  the*  prescribed  quantity  at  the  rate 
of  5,000,000  gallons  per  acre  daily,  and  the  total 
area  required  would  have  been  21.33  acres;  0.89 
acre  less  than  with  20  beds  designed  for  thirty-day 
periods,  and  0.66  acre  less  than  for  n  beds  also  de- 
signed for  thirty-day  periods.  If  a  different  length 
of  time  is  assumed  for  the  filter  to  be  out  of  service 
the  resulting  proportion  of  reserve  area  will  also  be 
slightly  changed. 

Location  and  Grouping  of  Beds. — After  having  de- 
termined the  proper  area  and  number  of  beds,  the 
grouping  of  the  beds  into  an  economical  design  will 
be  influenced  by  the  shape  of  the  available  tract  of 
land,  its  topographical  features,  and  the  judgment 
of  the  designer.  The  points  to  be  borne  in  mind  are: 
A  sufficient  area  must  be  reserved  for  the  washing 
and  storing  of  the  sand  during  cold  weather,  when 
washing  would  be  attended  with  considerable  diffi- 
culty and  expense;  and,  in  case  the  beds  are  not  cov- 
ered, for  sufficient  space  for  storing  ice  cut  to  per- 
mit cleaning;  to  allow  of  sufficient  room  for  the 
location  of  the  various  pipes  below  the  ground,  and 
the  tramways  above  the  ground  for  the  handling  of 
the  materials;  for  the  convenient  placing  of  the  filters 
relative  to  the  sand-court  so  as  to  make  the  average 
distance  that  the  sand  must  be  conveyed  as  short  as 


WATER  FILTRATION   WORKS. 


«,  INTAKE  FROM  LAKE. 

Z>,  FORE- BAYS 

C,  PUMPING  MACHINERY  FOR  DELIVERING 

WATER  ON  FILTERS. 
t7,  34  SAND  FILTERS. 
C,  4  FtLTERED-WATER  RESERVOIRS. 
/,  PUMPING  STATIONS  FOR  SENDING 

FILTERED  WATER  TO  CITY. 
gT,  SAND  WASHING  MACHINERY. 
7l,  ENGINEER'S  OFFICE. 
**,  LABORERS'  LUNCH  AND  WAITING  HALL. 
'7c,  RESIDENCES  AND  OFFICES  OF  THE 

OFFICERS  OF  THE  WORKS. 


L 


200 


SCALE  OF  METERS 

N 


L A  KE        M U a  0  EL 

FIG.  5.-— ARRANGEMENT  OF  THE  LAKE  MUGGEL  FILTER  PLANT, 
BERLIN,  GERMANY. 


DESIGNING   SLOW  SAND-FILTERS.  1 1 5 

possible,  and  the  proper  placing  of  the  clear-water 
reservoir  relative  to  the  filters  so  as  to  make  the 
length  of  piping  a  minimum.  The  arrangement  of 
the  Lake  Muggel  works  at  Berlin  is  shown  in  Fig.  5. 
Shape  of  Filter-beds. — Filters  are  usually  made 
rectangular  in  plan  when  the  topography  of  the  land 
does  not  require  some  other  shape.  Circular  or 
polygonal  shapes  are  rarely  used  when  the  rectangu- 
lar shape  is  possible,  although  in  very  small  covered 
filters  the  circular  form  is  quite  advantageous.  The 
principal  arguments  for  the  circular  shape  are  that 
with  it  the  cost  of  surrounding  walls  is  a  minimum 
for  a  given  area,  and  the  area  of  contact  between  the 
side-walls  and  the  sand  is  a  minimum,  thus  re- 
ducing the  danger  of  unfiltered  water  passing  down 
between  the  sand  and  walls  also  to  a  minimum.  A 
typical  plan  of  one  of  the  Berlin  filters  (Lake  Mug- 
gel)  is  shown  in  Fig.  6.  The  most  economical  shape 
for  a  rectangular  filter-basin,  if  not  subdivided,  is  the 
square.  If  divided  into  several  basins  the  economi- 
cal dimensions  may  be  obtained  from  the  following 
formulas,  in  which  it  is  assumed  that  the  dividing 
walls  cost  about  the. same  per  foot  run  as  the  side- 
walls. 

If  the  basins  are  all  in  one  row,  side  by  side,  the 

I  _ 

length  of  the  short  side,  *=  -^--7,  in  which  3;  is 

the  length  of  the  long  side  and  n  the  number  of  fil- 
ters. If  the  filters  are  placed  in  two  rows,  back  to 
back,  and  side  by  side,  in  the  row,  the  formula  be- 


n6 


WATER  FILTRATION    WORKS. 


FIG.  6.— PLAN  OF  BERLIN  (.MUGGEL)  FILTER-BED. 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      II? 

comes,  x  =  — -,  if  there  are  the  same  number  of 

beds  in  each  row. 

Depth  of  Filters. — The  depth  should  be  sufficient 
to  provide  for  the  filtering  materials  and  the  superin- 
cumbent water;  it  will  generally  be  about  10  feet, 
varying  a  foot  or  two  each  way  from  this  in  special 
cases,  depending  upon  the  fineness  of  the  filter-sand, 
the  character  of  the  water  and  the  economies  of 
design. 

CONSTRUCTION. 

Preparation  of  Site. — Where  the  ground-water 
level  is  higher  than  the  bottoms  of  the  proposed  fil- 
ters, the  site  should  be  drained  by  a  system  of  pipes, 
laid  with  open  joints,  and  surrounded  with  gravel,  so 
that  after  the  filters  are  completed  there  can  be  no 
upward  pressure  on  their  bottoms.  The  drains 
should  discharge  at  an  outfall,  or  into  a  sump,  or 
well,  from  whidh  the  water  may  be  pumped.  The 
site  of  the  Hamburg  filters  is  underdrained  in  this 
way,  the  sub-soil  water  being  pumped  from  a  well 
and  discharged  into  t'he  river. 

Side  Slopes  and  Bottoms. — Open  filters  are  fre- 
quently built  with  sloping  side-walls,  formed  in  ex- 
cavation or  by  embankment,  rather  than  with  verti- 
cal retaining  walls.  The  side  slopes  are  usually  i 
to  2  or  i  to  3,  and  are  protected  in  various  ways. 
Generally  a  layer  of  well-packed  clay  provides  for 
water-tightness,  the  surface  being  protected  by  a 
paving  of  brick,  stone,  or  concrete.  The  Hamburg 


Il8  WATER  FILTRATION    WORKS. 

filters  are  excavated  with  side  slopes  of  I  to  2.  The 
bottoms  and  slopes  are  covered  with  puddled  clay. 
The  bottom  is  paved  with  a  floor  of  bricks  laid  on 
their  sides,  and  the  slopes  with  bricks  set  on  edge;  in 
both  cases  laid  in  cement  mortar.  In  using  this 
method  of  lining  very  hard  impervious  bricks  are  de- 
sirable. In  the  zone  where  there  is  danger  of  ice 
forming  and  adhering  to  the  sides,  every  precaution 
should  be  taken  to  make  the  paving  impervious  and 
able  to  resist  frost  and  abrasion.  Open  beds  with 
sloping  side-walls  present  the  advantage  that  they 
are  not  so  apt  to  be  damaged  with  frost  and  ice  as  are 
those  with  vertical  walls.  The  beds  with  sloping  sides 
are  said,  however,  to  be  the  more  difficult  to  keep 
water-tight.  All  square  corners  should  be  avoided 
in  the  construction  of  beds  with  sloping  side-walls, 
as  there  is  great  danger  of  the  formation  of  cracks 
along  the  angles,  which  would  allow  the  water  to  per- 
colate to  the  underdrains  without  being  properly  fil- 
tered. The  relative  costs  of  open  beds  with  sloping 
and  those  with  vertical  side-walls  will  depend  upon 
circumstances.  Usually  beds  with  sloping  sides  will 
be  the  cheaper,  but  if  land  is  very  expensive  it  might 
be  possible  that  those  with  vertical  walls  would  be 
preferable. 

When  the  ground  upon  which  the  filters  are  to  be 
built  is  compressible  and  yielding,  many  difficulties 
may  be  encountered  in  holding  the  excavation  and 
the  walls.  In  such  cases  foundation  piles  under  the 
walls  and  piers,  and  sheet  piles  around  the  edges  of 
the  excavation,  or,  perfiaps,  the  construction  of  the 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      1 1 9 

side-walls  in  trenches,  followed  by  the  excavation  of 
the  interior  space,  or  the  dividing"  of  the  excavation 
into  different  sections  may  be  necessary. 

Precautions  to  Prevent  Water  Passing  to  the  Under- 
drains  in  an  Unaltered  State. — The  greatest  care 
should  be  taken  to  secure  perfectly  water-tight  work- 
manship, so  that  there  may  be  no  possibility  of  un- 
filtered  ground-water  finding  its  way  up  through  the 
bottom  and  into  the  underdrains. 

Sharp  salient  and  re-entrant  angles  of  all  piers, 
buttresses  and  side-walls  should  be  rounded  off  to 
insure  better  contact  between  the  sand  and  the  ma- 
sonry, to  preclude  the  danger  of  the  water  following 
such  angles  to  the  bottom  of  the  filter  without  being 
properly  purified.  In  order  to  prevent  the  unfiltered 
water  from  creeping  between  the  side-walls  and  sand 
it  would  be  well,  in  concrete  construction,  to  batter 
the  walls,  piers  and  buttresses,  from  the  bottom  to 
above  the  level  of  the  filtering  materials,  so  that  the 
settling  of  the  sand  under  the  action  of  the  water 
would  tend  to  make  the  contact  closer  the  longer  the 
filter  is  in  use.  In  the  Albany  filters  Mr.  Hazen  in- 
troduced ledges  around  the  faces  of  the  walls  and 
piers,  below  the  surface  of  the  sand,  formed  by  steps 
or  offsets  in  the  brick  work.  Mr.  Rudolph  Hering 
has  suggested  the  sanding  of  the  concrete  surface  be- 
fore the  mortar  has  set. 

Effects  of  Hot  Sun  on  Open  Filters. — Serious  leaks 
due  to  cracking  of  the  underlying  clay-puddle  are  apt 
to  occur  in  open  beds,  when  they  are  exposed  to  the 
hot  sun  for  several  days  with  the  water  drawn  off 


120  WATER   FILTRATION    WORKS. 

and  the  filtering  materials  removed.  Under  such 
conditions  there  may  also  occur  a  buckling  of 
the  floor  and  side-walls,  or  a  formation  of  cracks, 
resulting  in  decreased  efficiency  of  the  filters.  These 
dangers  are  entirely  avoided  by  covering  over  the 
beds  with  a  roof,  carried  on  piers,  and  overlaid  with 
a  few  feet  of  earth.  In  cold  climates  this  covering 
is  doubly  necessary  to  prevent  the  formation  of  ice 
of  considerable  thickness  upon  the  surface  of  the 
water,  the  removal  of  which,  by  disturbing  the  top  of 
the  sand  and  by  pre-requiring  the  greater  part  of  the 
water  to  be  filtered  upon  the  limited  area  that  can 
be  kept  properly  scraped,  may  reduce  the  efficiency 
very  greatly. 

Covering  Filters. — It  cannot  be  said  that  there  are 
any  advantages  to  be  gained  from  covering  filters, 
excepting  to  avoid  the  difficulties  inherent  to  keep- 
ing the  filters  operating  properly  during  cold 
weather,  and  to  prevent  the  growths  of  algae,  which 
produce  rapid  surface  clogging  on  the  filters  in  sum- 
mer weather.  To  prevent  the  latter  trouble,  a  light, 
inexpensive  trussed  roof  would  suffice,  as  its  only  ob- 
ject would  be  to  exclude  light.  Covered  and  uncov- 
ered filters,  other  things  being  equal,  yield  equally 
good  results,  when  properly  operated. 

Open  filters  are  more  .easily  cleaned  than  those 
with  covers,  and  have  the  advantage  of  presenting 
an  unbroken  surface  for  the  filtration  of  the  water. 
Covered  filters  have  many  columns,  piers,  buttresses, 
etc.,  which  pass  through  the  filtering  materials,  and 
around  which  it  is  difficult  to  place  the  sand  with  the 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      121 

same  degree  of  compactness  as  in  other  portions  of 
the  filter.  On  account  of  the  space  taken  up  by 
these  piers  and  buttresses  additional  area  is  required 
to  compensate  therefor.  It  is  also  said  that  for  some 
kinds  of  water  it  is  difficult  to  secure  sufficient  venti- 
lation in  covered  filters  during  summer.  This,  how- 
ever, seems  to  be  a  small  point. 

Generally  speaking,  therefore,  covering  will  be 
necessary  in  climates  where  long  periods  of  severe 
cold  are  likely  to  occur  in  the  winter,  or  where  algae 
growths  would  seriously  interfere  with  the  economic 
operation  of  the  plant  in  the  summer.  In  cities 
where  the  question  becomes  an  economic  one,astudy 
s'hould  be  made  of  the  number  of  successive  days  of 
freezing  weather,  the  degree  of  cold  during  these 
spells  and  the  lengths  of  the  periods  of  intervening 
thaws.  The  formation  of  ice  on  the  water  will  not, 
per  se,  affect  the  efficiency  of  filtration.  If  the  ice 
does  not  last  longer  than  the  period  during  which  the 
filters  can  be  safely  operated  between  cleanings,  it 
need  not  be  considered  as  a  factor  in  the  question  of 
providing  covers.  Since  covered  filters  cost  from  50 
to  100  per  cent,  more  than  the  open  type,  it  may,  in 
some  cases,  be  cheaper  to  provide  more  area  of  open 
niters  than  to  cover  those  actually  required,  if  by 
that  means  the  plant  can  be  operated  a  sufficiently 
long  time  to  allow  the  ice  to  melt  or  be  safely  re- 
moved from  part  of  the  area  before  scrapings  are 
necessary.  In  rather  mild  climates  a  trussed  roof, 
similar  to  that  over  the  Koenigsberg  filters,  might 
afford  sufficient  protection,  or  some  of  the  beds 


122  WATER    FILTRATION   WORKS. 

might  be  covered  and  some  left  uncovered,  as  at 
Stralsunder. 

Another  combination  which  might  be  advantage- 
ous in  climates  where  ice  would  give  trouble,  would 
be  to  provide  a  certain  proportion  of  the  required 
filter  capacity  in  open  slow  filters,  and  the  remainder 
in  rapid  filters  with  a  comparatively  low  rate,  say 
100,000,000  gallons  per  acre  per  day.  Then  during 
very  cold  weather  the  slow  filters  could  be  operated 


FIG.  7. — GROINED  ARCHES. 

at  a  slow  rate,  perhaps  half  of  the  summer  rate,  so  as 
to  postpone  the  times  of  scrapings,  and  the  rapid 
filters  could  be  operated  somewhat  more  rapidl  • 
than  at  the  summer  rate.  This  presupposes  that  the 
water  is  of  a  character  to  be  successfully  treated  with 
rapid  filters.  The  flexibility  of  rapid  filters,  within 
pretty  wide  limits,  as  noted  in  chapter  III.,  makes 
such  a  combination  as  this  quite  practical,  and  in 
some  cases  may  permit  the  building  of  open  slow 
sand-filters,  the  whole  plant  being  very  much  less 


CONSTRUCTION  OP  SLOW'  SAN&-FILT&RS.    12$ 

expensive  than  one  consisting  entirely  of  covered 
slow  sand-filters,  while  at  the  same  time  being  equally 
efficient.  During  the  winter  time,  the  relative  pol- 
lution of  most  streams  is  lower  than  in  summer,  be- 
cause the  polluting  matter  is  retained  longer  on  the 
surface  of  the  ground,  and  the  stream  flow  is  also 


FIG.  8. — MASONRY  GROINED  ARCHES  WITH  ARCH  RIBS. 

generally  greater  than  during  the  summer  months. 
Thus  in  the  summer  the  main  reliance  would  be  upon 
the  slow  sand-filters,  and  during  the  winter  upon  the 
rapid  sand-filters. 

In  very  cold  climates  the  cost  of  removing  the  ice 
is  a  significant  part  of  the  cost  of  operation  of  open 


124  WATER  FILTRATION   WORKS. 

filters.  The  tendency  in  the  German  works  is  tow- 
ard covered  filters,  while  in  England  and  Holland 
the  filters  are  almost  without  exception  uncovered. 
Groined  arches  (Fig.  7,  8  and  9),  springing  from 
the  tops  of  columns,  are  generally  used  for  covering 
filter-beds,  because  of  the  ease  with  which  they  may 


FIG.  9. — CONCRETE  GROINED  ARCHES. 

be  constructed  of  brick  masonry  or  concrete.  An 
interior  view  of  the  Ashland,  Wis.,  covered  slow  sand- 
filter  plant,  designed  by  Wm.  Wheeler,  C.E.,  is 
given  in  Plate  V.  The  roof  over  this  filter  is  the  first 
application  in  the  United  States  of  groined  arch  con- 
struction for  a  filter  cover. 


UNIVERSITY 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      12? 

The  average  thickness  of  the  groined  concrete 
arches  covering  the  Albany  filters  is  about  7  inches, 
the  thickness  at  the  crown  being  6  inches. 

The  new  Berlin  filters  have  covers  of  a  unique  de- 
sign. The  roof  is  a  series  of  domes,  supported  on 
piers  (Figs.  8  and  10).  The  domes  were  constructed 


FIG.  10. — DOMED  COVERING  WITH  ARCH  RIBS. 


by  springing  arch  rings  from  the  tops  of  the  piers, 
on  the  sides  and  diagonals  of  each  panel,  and  then 
filling  in  the  space  between  the  rings  with  the  shell 
of  a  dome,  the  brick  being  put  in  place  by  hand, 
without  the  use  of  centres.  The  work  is  beautifully 
done  and  is  very  effective,  although  much  more  ex- 
pensive than  concrete  groined  arches  would  have 
been.  They  have  a  way  of  doing  things  in  Europe, 
particularly  in  Germany,  in  the  building  of  public 
works,  which  might  well  be  emulated  by  American 


128  WATER  FILTRATION  WORKS. 

cities,  to  a  certain  extent,  at  least.  In  our  average 
American  town  the  policy  is  usually  to  be  over-or- 
nate in  structures  showing  above  ground,  and  to  be 
over-economical  in  the  execution  of  works  which  are 
out  of  sight.  Very  often,  however,  this  policy  is  not 
truly  economical.  In  works  pertaining  to  public  sani- 
tation nothing  can  be  too  good  that  will  conduce  to 
the  greater  care  and  attention  which  attractive  sur- 


FIG.  ii. — CYLINDRICAL  ARCHES. 

foundings  will  naturally  beget.  Nothing  is  more  con- 
ducive to  good  maintenance  than  appropriate  con- 
struction and  well-built  structures.  For  this  reason, 
in  filter  plants,  the  effort  should  be  made  to  have  the 
interior  finish  of  the  walls,  piers  and  arches  properly 
carried  out,  and  the  gate-houses,  entrances  and  other 
works  above  ground  architecturally  presentable. 
Few  engineers  are  skilful  enough  designers  to  be 
trusted  with  the  treatment  of  the  architectural  fea- 
tures. The  countless  monstrosities  in  the  shape  of 
pumping  stations,  etc.,  that  are  to  be  seen  in  our 
large  cities  bear  witness  to  the  folly  of  entrusting 


CONSTRUCTION  OF  SLOW  SAND-FILTERS. 

such  designs  to  men  educated  highly,  no  doubt,  in 
the  uses  for  which  the  structures  are  built,  but  incom- 
petent architecturally  and  artistically. 

Domed  constructions,  cylindrical  arches  (Fig.  n) 
or  composite  roofs  of  steel  and  concrete  may  be  used 
instead  of  groined  arches,  if  desirable.  An  eco- 
nomical form  of  roof  is  one  composed  of  flat  domes 
resting  on  the  tops  of  piers  (Figs.  12  and  13)  similar 


FIG.  12. — FLAT  DOMES. 

to  the  Berlin  roof,  above  described,  but  made  of  con- 
crete and  expanded  metal.  The  cost  of  making  the 
centering  for  this  form  is  a  trifle  more  than  for 
groined  arches,  but  the  saving  in  concrete  is  very 
considerable.  A  view  of  the  centering  for  the  groined 
arches  forming  the  roof  of  the  Somersworth,  N.  H., 
filter  is  given  in  Plate  VI. 

By  the  use  of  the  domed  construction  with  ex- 
panded metal,  properly  placed,  the  average  thick- 
ness Can  be  reduced  considerably  below  that  required 
for  groined  arches.  The  author  has  built  a  circular 
reservoir  for  spring  water,  now  in  service,  covered 


130  WATER  FILTRATION  WORKS. 

with  a  concrete  and  expanded-metal  dome,  of  a  span 
of  16  feet,  rise  of  2  feet,  and  average  thickness  of 
5  inches,  with  a  covering  of  2  feet  of  earth. 
The  earth  covering  was  dumped  on  it  from  wagons. 
He  has  also  built  two  other  domes,  in  a  similar  man- 


FlG.  13. — CONCRETE  DOMED  CONSTRUCTION. 

ner,  having  spans  of  20  feet,  rise  of  3  feet  and  aver- 
age thickness  of  5  inches. 

Drainage  of  Roof. — The  water  falling  on  the  roofs 
of  filters  may  be  allowed  to  drain  into  the  filters 
through  the  roof,  in  pipes  carried  down  through  the 
piers  and  discharging  above  the  level  of  the  sand. 
The  top  ends  of  these  drains  should  be  covered  in 
some  way  so  as  to  prevent  the  entrance  of  dirt,  and 
should  provide  free  exit  for  the  water,  so  as  to  pre- 
vent injury  to  the  work  by  the  action  of  frost. 

The  roof  should  be  covered  with  coarse  sand,  or 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      133 

gravel,  to  facilitate  drainage,  and  on  top  of  the 
gravel  about  two  or  three  feet  of  earth  should  be 
spread  to  keep  out  the  cold.  The  top  six  inches  of 
filling  should  be  top-soil,  which  should  be  fertilized 
and  seeded  with  grass,  while  the  slopes  of  terraces 
or  banks  should  be  sodded.  The  treatment  of  the 
tops  of  covered  filters  offers  opportunities  for  the 
display  of  taste  in  landscape  work. 

Ventilation. — Much  more  provision  should  be 
made  for  ventilation  and  lighting  than  is  usual  in 
reservoir  construction,  as  the  operations  of  cleaning 
and  refilling  filters  will  occur  quite  frequently  and 
can  only  be  done  effectively  in  good  light. 

In  the  centre  of  alternate  panels  in  the  roof  man- 
holes should  be  built  to  provide  for  ventila- 
tion and  light.  These  should  extend  from  the  roof 
to  the  top  of  the  earth  covering,  and  should  be  about 
two  feet  in  diameter  at  the  top  and  slightly  larger 
at  the  bottom.  Each  should  be  provided  with  a 
cover  which  could  be  removed  if  necessary.  A  con- 
venient and  satisfactory  arrangement  is  to  have  a 
double  cover,  the  lower  one  being  wire-glass  and  the 
upper  one  of  metal  treated  with  a  preservative  coat- 
ing. 

In  large  plants  it  is  desirable,  if  not  always  neces- 
sary, to  install  an  electric-lighting  plant,  with  arc 
lights  for  the  sand-courts,  roads,  etc.,  and  incandes- 
cent lights  placed  in  the  gate-houses,  and  distributed 
through  the  basins  of  covered  filters. 

There  is  no  reason  why  the  roof  need  be  much 
above  the  highest  water-level,  though  sufficient 


134  WATER  FlLTRAl^ION    WORKS. 

head-room  should  be  provided,  of  course,  for  the 
convenience  of  the  workmen  in  cleaning.  The  actual 
height  will  depend  upon  the  depth  of  water  allowed 
upon  the  filter  surface,  the  limiting  filtration  head 
and  special  features  of  the  regulating  apparatus  and 
conduit  leading  to  the  filtered-water  reservoir. 

Tramways  for  Sand  Haulage. — In  nearly  all  the 
large  slow  sand-filter  plants  now  in  operation  it  is  the 
general  practice  to  provide  tramways  for  transport- 
ing the  sand  removed  from  the  beds  to  and  from  the 
sand  washers.  It  will  not  be  long,  however,  before 
some  radical  changes  will  be  effected  in  the  methods 
of  cleaning  slow  sand-filters,  having  for  their  object 
the  reducing  of  the  amount  of  hand  labor  involved  in 
the  process  as  now  practised.  When  tramways  are 
used  it  is  convenient  to  provide  branches  from  the 
main  tracks,  one  running  into  each  covered  filter  and 
sloping  down  to  the  level  of  the  sand  surface,  so  that 
the  cars  can  be  taken  in  and  out  of  the  filters.  The 
track  should  be  supported  between  two  rows  of  piers 
and  extend  generally  to  about  the  centre  of  the  filter. 
Over  the  track  the  roof  is  usually  a  cylindrical  arch, 
its  axis  sloping  with  the  track  so  as  to  provide  suffi- 
cient head-room. 

For  open  filters  portable  tracks  are  used  with  suc- 
cess. 

Bottoms. — Inverted  groined  arches,  Fig.  14,  make 
the  best  form  of  bottom  for  slow  sand-filters,  because 
this  form  is  economical,  furthers  the  proper  distribu- 
tion of  the  load  on  the  columns  carrying  the  roof, 
gives  a  strong  section  and  provides  valleys  in  which 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      137 


UNDERDRAIN  CONCRETED 

SECTION  A-B  CONCRETE  VAULT 


XONCRETE. 


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SCALE  OF  FEET 


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FIG.  14.— TYPICAL  PLAN  AND  SECTIONS  OF  COVERED  SLOW 
SAND-FILTER, 


138 


WATER  FILTRATION    WORKS. 


the  underdrains  may  be  laid.  Slow  sand-filter  floors 
should  never  be  horizontal  planes,  but  should  be 
broken  into  ridges  and  valleys  inclining  towards 
the  underdrains  so  as  to  remove  the  water  as 


3    D 

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§  a 


OVERFLOW  AlTD  DRAINT*] 


EGULATING.APPARATUS 


UJJJNFILTERED  WATER 


FIG.  15.—  PLAN  OF  FILTER-BED,  ZURICH,  SWITZERLAND. 

fast  as  it  is  filtered.  The  manner  of  accomplish- 
ing this  in  the  Zurich  filters  is  shown  in  Fig. 
15.  If  allowed  to  stand  in  the  gravel  the  water  will 
gradually  deteriorate  in  quality,  to  a  greater  or  lesser 
degree.  The  best  practice,  in  regard  to  the  under- 
drains for  collecting  the  water  after  it  has  passed 
through  the  sand  and  gravel,  is  to  use  small  vitrified 
pipes  for  the  lateral  drains,  placing  them  upon  the 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      139 

filter  floor,  and  form  the  main  collector  in  the  con- 
crete bottom  of  the  filter.  This  main  collector  should 
have  a  semi-circular  invert  and  straight  vertical  side- 
walls  and  should  be  covered  with  slabs  of  concrete 
or  stone. 

This  method  is  better  than  using  pipe,  built  into 
the  concrete,  for  the  main  collector,  because  during 
the  construction  of  the  basins,  mortar,  dirt  and  other 
debris  is  likely  to  get  into  the  main  underdrain,  and 
its  cleaning  later  may  be  a  difficult  matter.  With 
the  open  drain,  'however,  the  debris  can  be  easily 
removed. 

Underdraws. — The  sizes  of  the  underdrains  depend 
upon  the  area  of  the  bed,  the  distance  between  the 
collectors  and  the  amount  of  water  to  be  fil- 
tered in  a  given  time.  In  proportioning  the  sizes, 
ample  allowance  should  be  made  on  the  side  of 
safety,  so  that  the  frictional  resistances  may  not 
cause  unequal  rates  of  filtration  in  different  parts  of 
the  beds.  As  the  cost  of  the  underdrains  is  a  very 
insignificant  part  of  the  cost  of  filter-beds,  it  is  bad 
practice  to  attempt  to  keep  the  sizes  down  to  the 
danger  limit,  to  save  a  few  hundred  dollars,  at  the 
risk  of  lessening  the  efficiency  of  the  filters.  I  would 
suggest  that  if  the  sizes  are  proportioned  so  that  the 
total  frictional  resistance,  when  filtering  at  the  maxi- 
mum rate,  from  the  outlet  to  the  most  distant  point 
is  kept  down  to  about  .01  to  .02  foot,  no  trouble 
will  be  experienced. 

Great  care  should  be  taken  in  placing  the  under- 
drains, if  of  pipes,  to  leave  enough  space  open  at  the 


140 


WATER  FILTRATION    WORKS. 


joints  to  permit  the  water  to  enter  without  requiring 
too  great  velocity  head. 

The  conversion  diagrams  (Figs.  16,  17  and  18)  will 


GALLONS  PER  SECOND. 


85,000 


,         ,         i         i 

§       I       i       1       1       i       I       1 

GALLONS  PER  DAY"  ' 
FIG.  16. — CONVERSION  DIAGRAM. 

Gallons  per  day  into  cubic  feet  per  second  and  per  minute,  and 
gallons  per  second. 

be  of  service  in  estimating  the  quantities  of  water  that 
will  be  discharged  by  the  underdrains  and  collectors. 

For  convenience  in  proportioning  the  sizes  of  pipe 
underdrains,  Fig.  19,  based  on  Kutter's  Formula, 
with  ^  =  .013,  has  been  prepared.  Knowing  the 
quantity  of  water  to  be  filtered,  and  the  allowable 
loss  of  head,  the  sizes  and  slopes  can  readily  be  found. 

In  place  of  using  pipes  for  underdrains  some  works 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      H1 

have  floors  made  of  two  layers  of  ordinary  bricks, 
the  bottom  bricks  resting  on  edge,  a  little  distance 


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apart,  and  the  top  layer  lying  flat  upon  them  to  form 
the  floor  for  the  filtering  materials.    In  other  places 


142 


WATER  FILTRATION   WORKS. 


special  hollow  bricks  are  used  for  the  purpose.  The 
form  of  bricks  used  at  Zurich  is  shown  in  Fig.  20. 
There  is  no  special  advantage  to  be  gained  by  this 


10        9 


MILLION  GALLONS  PER  ACRE  PER  DAY. 
8765432 


2345G78910 

MILLION  GALLONS  PER  ACRE  PER  DAY..^ 
FIG.  18. — CONVERSION  DIAGRAM. 

Million   gallons  per  acre  per  day  for  different  areas  into  cubic 
feet  per  second. 

form  of  construction,  and  its  cost  is  considerably  in 
excess  of  the  more  simple  expedient  of  using  vitri- 
fied pipes  and  properly  graded  and  placed  gravel 
layers. 

Gravel  Layers. — To  provide  free  passage  laterally 
to  the  underdrains  it  is  the  custom  to  cover  the  floor 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      143 


with  layers  of  gravel  or  broken  stone  of  sufficient 
thickness  to  permit  free  passage  of  the  water  without 
consuming  too  much  friction  head.  The  resistance  to 

LOSS  OF  HEAD,  IN  FEET,  IN  A  LENGTH  OF  TEN  FEET. 


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FIG.  19. — DIAGRAM  SHOWING  FRICTIONAL  Loss  OF  HEAD  IN 
PIPES. 

the  motion  of  the  water  depends  upon  the  size  of  the 
particles  of  the  gravel,  the  rate  at  which  the  water  is 
passed  through  the  gravel,  the  temperature  of  the 


144 


WATER  FILTRATION   WORKS. 


water,  and  the  thickness  of  the  gravel  layer.  In  Fig. 
21  the  data  given  in  the  report  of  the  Massachusetts 
State  Board  of  Health  for  1892  are  arranged  in  such 
a  manner  that  the  loss  of  head  in  a  gravel  layer  one 
foot  thick  can  be  taken  by  inspection,  for  various 
sizes  of  gravel  and  distances  that  the  water  must 


FIG.  20. — HOLLOW  FLOOR,  ZURICH  FILTERS. 

travel  to  reach  the  underdrains,  for  a  rate  of  filtration 
of  one  million  gallons  per  acre  per  day.  For  other 
thicknesses  of  gravel  the  rate  will  vary  inversely  as 
the  thickness,  and  for  other  rates  of  filtration  di- 
rectly as  the  rate. 

The  gravel  should  be  placed  in  the  filters  in  con- 
tinuous layers,  the  particles  of  each  layer  being  a 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      145 

little  smaller  than  those  of  the  layer  below,  to  pre- 
vent the  filtering  sand  from  being  washed  into  the 
drains.  The  coarse  gravel,  or  broken  stone,  bed  is 
to  be  considered  only  as  serving  the  purpose  of  per- 
mitting the  more  or  less  free  movement  of  the  water 
to  the  underdrains.  The  superimposed  thin  layers  of 
gravel  of  decreasing  sizes  are  to  support  the  sand 


ill 


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\ 

\ 

\ 

s 

\ 

\ 

^ 

- 

^ 

FIG. 


THICKNESS  Of  GRAVEL  LAYER.,  ONE  FOOT  :  FOR  OTHER  THICKN.ESSES  TH.E.  LOSS 

OF  HEAD  WILL  VARY  INVERSELY  AS  THE  THICKNESS. 

\  RATE  OF  FILTRATION,  1  000000  GALLONS  PER  ACRE  DAfLY:  FOR  OTHER  RATEB 

.  THE  LOSS  OF  HEAD  WILL  VARY  DIRECTLY  AS  THE  RATE. 

2i.— DIAGRAM   SHOWING    HEAD  OF  WATER   CONSUMED    IN 
PASSING  HORIZONTALLY  THROUGH  GRAVEL  LAYERS. 


and  prevent  it  from  being  carried  into  the  under- 
drains by  the  sinking  of  the  water. 

These  layers  need  not  be  thick,  but  they  should 
be  of  screened  gravel,  and  each  layer  should  be  con- 
tinuous over  the  one  below  it.  When  properly  placed 
and  graded  as  to  sizes,  experience  has  shown  that 
there  is  very  little  movement  of  the  particles,  and 
layers  ii  to  2  inches  thick,  depending  upon  the  size 
of  the  particles,  have  been  found  to  be  in  perfect  con- 
dition after  several  years  of  service.  No  difficulty 
will  be  experienced  if  care  is  taken  that  the  particles 


146  WATER  FILTRATION    WORKS. 

in  each  layer  are  not  more  than  three  or  four  times 
as  large  as  the  particles  in  the  superimposed  layer. 
An  interior  view  of  the  Somersworth,  N.  H.,  slow 
sand-filters,  taken  when  the  underdrains  and  gravel 
layers  were  being  placed  in  position,  is  given  in  Plate 
VIII. 

The  influence  of  the  size  of  the  particles  and  the 
thickness  of  the  layers  will  be  felt  in  the  head  used 
up  in  the  passage  of  the  water  to  the  underdrains. 
The  smaller  the  gravel  and  the  thinner  the  layer  the 
greater  the  head  necessary  to  pass  a  given  quantity 
of  water  in  a  given  time.  This  may  result,  with  poor 
designing,  in  the  parts  of  the  filters  remote  from 
the  drains  passing  the  water  at  a  slower  rate  than 
parts  over  the  drains,  a  condition  which  should  be 
avoided  as  much  as  possible.  Since  the  thickness  of 
the  gravel  layer  must  be  deeper  the  greater  the  dis- 
tance between  the  underdrains,  there  can  be  found 
an  economical  depth  of  gravel  when  the  size  of  its 
particles,  its  cost,  the  rate  at  which  the  water  is  to  be 
delivered,  and  the  allowable  loss  of  head  are  known. 
-  Filtering-sand. — The  velocity  with  which  water 
will  pass  through  sand  layers  of  different  effective 
sizes,  at  different  temperatures,  and  under  different 
heads,  has  been  the  subject  of  experiment  by  the 
Massachusetts  State  Board  of  Health  at  different 
times.  The  first  experiments  were  summarized  in 
the  report  for  1892.  These  results  were  expressed 
by  the  formula: 

h  (t  Fahr.  +  io°\ 


CONSTRUCTION  OF  SLOW  SAND-FtLTEttS.      149 


velocity  of  the  water  in  a  solid  column  of  the 
same  area  as  that  of  the  sand,  in  meters,  daily, 
or  approximately,  in  million  gallons  per  acre 
daily. 

r  —  a  constant;  its  value  for  clean  sands  is  about 
1,000,  and  for  filters  that  have  been  some  time 
in  service  it  is  about  800. 

d  =  the  effective  size  of  the  sand  grain  in  millimeters. 
/i  =  the  loss  of  head  due  to  passing  through  the  sand 

at  the  given  rate. 
/  =  the  thickness  of  the  sand  layer. 
/  —  the  temperature  of  the  water  in  degrees  Fahren- 

heit. 

The  formula  only  'applies  when  the  pores  of  the 
sand  are  entirely  filled  with  water,  when  the  sand  is 
well  compacted,  and  when  there  is  no  clogging  of 
the  pores.  It  is  also  applicable  only  in  the  case  of 
sands  from  o.io  to  3.00  mm.  in  effective  size,  and 
with  uniformity  coefficients  lower  than  5. 

From  this  formula  the  loss  of  head,  h,  can  readily 
be  found  if  the  rate  of  filtration,  the  effective  size  of 
the  sand  and  the  depth  of  the  filtering  materials  are 
given. 

Depth  of  Sand.  —  The  effect  of  the  size  of  the  sand 
grain,  uniformity  coefficient,  depth  of  the  sand  layer 
and  the  rate  of  filtration,  on  the  efficiency  of  the  pro- 
cess of  slow  sand-filtration  were  fully  discussed  in 
Chapter  III. 

The  usual  depth  of  sand  in  the  European  filters 
is  from  2  to  3  feet,  but  in  most  of  them,  however, 
the  gravel  layers  under  the  sand  are  very  much 


WATER  FILTRATION  WORKS. 

thicker  than  necessary.  It  is  advisable  to  make  the 
gravel  layers  as  thin  as  would  be  safe,  in  order  that 
the  total  depth  of  the  filter  may  not  be  unduly  in- 
creased. The  proper  depth  for  the  sand  will  vary  ac- 
cording to  its  character  and  the  character  of  the  wa- 
ter. Very  coarse  sands  require  thick  beds,  while  very 
fine  sands  do  not  require  so  great  a  thickness.  The 
best  thickness  can  only  be  determined  from  a  study 
of  the  sand  and  the  results  that  must  be  obtained. 
Generally,  with  ordinary  sands,  such  as  would  be 
called  good  mortar  sands,  and  ordinary  waters, 
troubled  neither  with  excessively  fine  clay  turbidity 
nor  algae  growths,  a  depth  of  five  feet  is  best.  With 
finer  sands  four  feet  may  be  sufficient. 

Character  of  Sand. — The  sand  should  be  free  from 
clay,  loam  and  vegetal  matter,  and  preferably  also 
free  from  particles  of  limestone  and  other  mineral 
matter  that  might  affect  the  water  injuriously.  The 
uniformity  coefficient  should  be  as  low  as  possible, 
and  the  sand  grains  'hard  and  firm  so  as  not  to  disin- 
tegrate under  the  action  of  the  water.  Sands  con- 
taining lime  and  magnesia  will  render  the  water 
somewhat  harder  after  filtration.  Dirty  sands  should 
be  washed  to  remove  the  dirt,  before  being  placed  in 
the  filters,  as  such  matter  would  cause  clogging  and 
reduced  efficiency.  Particular  care  should  be  taken 
to  secure  sand  of  uniform  character  and  fineness,  be- 
cause if  several  different  sizes  are  used  in  the  same 
bed  they  will,  on  account  of  offering  different  resist- 
ances, cause  different  rates  of  filtration  in  different 
parts  of  the  bed.  Also,  if  the  uniformity  coefficient 


o  » 


THE 

' 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      1 53 

is  high  there  will  necessarily  be  a  good  deal  of  sand 
lost  during  washing,  as  the  velocity  necessary  to  wash 
the  larger  grains  may  be  great  enough  to  float  the 
finer  ones  away. 

Placing  Sand  in  Filters. — A  great  deal  of  care  is 
necessary  in  placing  the  sand  in  the  filters  to  secure 
uniform  density  of  the  entire  bed.  It  should  not  be 
deposited  in  one  layer,  nor  in  thin  layers,  each  spread 
out  and  levelled,  but  rather  in  two  or  three  layers, 
the  face  of  each  layer  being  stepped  back  several  feet 
behind  the  next  lower  one,  and  all  carried  continu- 
ously across  the  bed  of  the  full  thickness.  After  it  is 
in  place  the  top  should  be  levelled  off  to  a  flat  hori- 
zontal surface,  planks  being  placed  on  the  sand  for 
the  men  to  work  on,  so  that  their  boots  will  not  com- 
pact the  surface. 

A  view  of  the  Somersworth,  N.  H.,  covered  slow 
sand-filter,  taken  when  the  filtering-sand  was  being 
placed  in  position  in  three  layers,  is  given  in  Plate  X. 

Placing  the  Gravel. — In  placing  the  gravel  around 
the  underdrains  care  must  be  taken  to  see  that  it 
is  thoroughly  settled  before  the  sand  is  placed  in 
the  filter,  because  subsequent  settling  may  produce 
vertical  lamination  through  the  sand,  allowing  unfil- 
tered  water  to  pass  down  to  the  underdrains.  The 
gravel  should  be  deep  enough  to  bury  the  lateral  un- 
derdrains, and  should  cover  the  space  between  them. 
It  should  not,  however,  extend  clear  to  the  side- 
walls,  or  edges,  of  the  filters,  but  should  stop  3  or  4 
feet  from  the  walls  so  as  to  force  the  water  to  flow 
along  the  bottom  of  the  filter  under  the  sand,  as 


1 54  WATER  FILTRATION  WORKS. 

was  done  at  Albany  by  Mr.  Allen  Hazen.  If  the 
concrete  side-walls  are  made  smooth  when  first 
built,  and  are  then  washed  down  with  a  brush 
coat  of  neat  Portland  cement,  good  results 
will  be  obtained  in  preventing  the  too  rapid  pas- 
sage of  the  water  between  the  sand  and  wall  sur- 
face. As  stated  in  chapter  III.,  the  walls  and  piers 
should  be  battered  below  the  sand  line  so  that  the 
sand  will  settle  tightly  against  them;  this  provision 
ought  to  make  a  tight  joint  between  the  sand  and 
walls.  It  is  not  good  practice  to  plaster  the  inside 
of  the  walls  of  filters  below  the  sand  line  with  a  coat 
of  cement  applied  with  a  trowel,  because  such  coats 
frequently  adhere  in  spots  only,  leaving  spaces  be- 
hind through  which  the  water  can  flow  if  cracks 
should  develop  in  the  plastering.  Brick  walls  and 
piers  are  also  to  be  looked  upon  with  more  suspicion 
than  if  made  of  concrete,  because,  as  bricks  are  gen- 
erally laid,  the  mortar  cannot  be  depended  upon  to 
adhere  closely  to  the  bricks  in  the  vertical  joints,  and, 
therefore,  unfiltered  water  may  follow  cracks  and 
joints  to  the  bottom  of  the  filter.  The  stopping  of 
the  gravel  layer  a  few  feet  from  the  sides  of  the  fil- 
ters is  designed  to  correct  this  evil.  It  is  also  better 
to  finish  the  inside  plastering,  where  it  is  necessary, 
with  a  felt  float  rather  than  with  a  trowel,  as  hard  pol- 
ished neat-cement  surfaces  are  almost  sure  to  check 
after  setting. 

Sand  Washing. — In  case  the  sand  is  dirty  and  the 
uniformity  coefficient  too  high,  it  should  be  screened 
to  remove  the  large  pebbles,  and  then  washed  to  re- 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      1 57 

move  the  dirt.  The  sand  washers  in  common  use  by 
contractors,  consisting  of  revolving  screens  and  cylin- 
ders with  sprays  of  water  playing  through  the  sand, 
are  quite  effective  for  removing  the  clay  and  dirt. 
They  usually  have  a  helix  on  the  interior  of  the  cylin- 
der, which  works  the  sand  up  to  the  high  end,  dis- 
charging it  into  cars  after  it  is  washed.  The  dirty 
water  and  fine  particles  are  washed  out  at  the  lower 
end.  This  form  of  washer  is  in  use  in  Berlin.  An- 
other— and  for  some  reasons  a  better — washer  is  the 
form  now  popularly  known  as  the  Hamburg  washer, 
from  its  use  in  an  improved  form  at  the  Hamburg 
filters.  The  washer  consists  of  a  series  of  hoppers 
with  ejector  nozzles  perforating  the  bottom  of  each, 
and  pipes  and  troughs  arranged  so  that  the  sand 
from  each  hopper  is  lifted  to  the  next,  the  fine  dirt 
going  over  the  sides  of  the  hoppers  with  the  wash- 
water.  This  apparatus  is  shown  in  sketch  in  Figs. 
22,  23  and  24,  and  in  the  photographic  views,  Plates 
XIII  and  XIV,  and  is  the  form  now  used  almost  ex- 
clusively in  modern  filter  plants,  because  of  the  con- 
venience and  thoroughness  with  which  the  washing 
of  the  dirty  sand  removed  after  cleaning  filter-beds 
can  be  done. 

Sand  may  be  washed  quite  clean  with  a  hose  if 
other  apparatus  is  not  at  hand.  A  platform  should 
be  prepared  with  walls  around  all  sides,  and  a  mova- 
ble-board front.  The  bottom  of  the  platform  may  be 
of  wood  or  of  brick,  and  should  slope  toward  the 
open  end.  The  sand  is  placed  in  a  pile  at  the  high 
end  of  the  platform  and  the  water  played  on  it  from 


158 


WATER  FILTRATION   WORKS. 


Hi     Jim 


FIG.  22.— CROSS-SECTION  THROUGH  EJECTOR  SAND-WASHER. 


FIG.  23. — PLAN  OF  EJECTOR  SAND-WASHER. 


SCALE  OF  FEET 


FIG.  24.— LONGITUDINAL  SECTION  THROUGH  EJECTOR  SAND- 
WASHER. 


CONSTRUCTION-  OF  SLOW  SAND-FILTERS.      1 59 

the  hose.  The  water  and  dirt  overflow  the  weir  in 
front,  and  the  sand  remains  on  the  platform.  The 
sand  is  kept  thrown  back  as  the  washing  progresses. 
The  washing  has  been  carried  far  enough  w'hen  the 
wash-water  runs  clear.  The  washing  is  done  in  this 
way  at  Antwerp  and  at  some  of  the  London  filter 
plants. 

At  Edinburgh  the  sand  is  washed  in  boxes  hav- 
ing perforated  false  bottoms,  the  water  forced  up 
through  the  perforations  carrying  the  dirt  over  the 
edges  of  the  boxes.  Of  course,  in  all  plants  experi- 
ment is  necessary  to  determine  the  quantity  of  water 
necessary  to  properly  wash  the  sand,  and  the  force 
with  which  it  must  be  used,  so  as  not  to  carry  off  too 
great  a  proportion  of  the  finer  particles. 

The  cost  of  washing  sand,  and  the  quantity  of 
wash-water  required,  will  be  discussed  under  the 
operation  of  slow  sand-filters. 

Regulating  Apparatus. — Generally  arrangements 
are  made  for  keeping  the  surface  of  the  water  on  the 
filters  at  a  constant  height,  allowing  the  water  to  fall 
in  the  regulating  chamber  as  the  frictional  resist- 
ances increase  with  service.  This  is  accomplished 
by  placing  a  valve,  operated  by  a  float  resting  on  the 
surface  of  the  water  in  the  filter,  on  the  inlet  for  raw 
water.  In  some  works,  however,  the  water  surface 
in  the  regulating  chamber  is  kept  at  a  constant  level, 
and  the  depth  of  the  water  on  the  filters  is  allowed  to 
increase,  as  clogging  takes  place,  while  in  others  the 
water  in  both  the  regulating  chamber  and  the  filters 
is  allowed  to  fluctuate,  arrangements  being  made  to 


l6o  WATER  FILTRATION    WORKS. 

prevent  too  great  a  depth  in  the  filter  by  hand- 
regulation  of  the  inlet  valve  and  by  an  overflow. 
Examples  of  each  kind  are  to  be  found  in  the  well- 
known  filters  of  Europe.  To  the  first  class  belongs 
the  apparatus  used  at  Hamburg,  to  the  second  the 
older  Berlin  apparatus,  and  to  the  third  the  auto- 
matic devices  used  at  Warsaw  and  Zurich.  Many 
plants  have  no  special  apparatus  for  regulating  the 
height  of  water  on  the  filters,  but  are  worked  by 
opening  or  closing  a  valve  on  the  feed-pipe,  by  hand, 
in  accordance  with  the  necessities  of  service.  This 
is  the  case  at  Zurich,  on  the  earlier  Berlin  filters,  and 
is  the  general  English  practice.  At  Hamburg  and 
Leeuwarden,  the  new  filters  at  Berlin,  the  Albany 
filters,  and  at  several  other  places  the  depth  is  auto- 
matically limited  by  a  float  upon  the  surface  of  the 
water  on  the  filters.  This  float  opens  and  closes  a 
valve  on  the  inlet  pipe.  Care  should  be  taken  to 
provide  some  sort  of  stilling  chamber  around  the 
float,  so  that  it  may  not  be  thrown  out  of  line  and 
thus  jam  the  valve  and  cause  it  to  become  inopera- 
tive. 

In  passing  through  the  filters  a  certain  amount  of 
head  is  used  up  in  forcing  the  water,  with  the  proper 
velocity,  through  the  sand  and  underdrains.  This 
loss  of  head  increases  with  the  length  of  time  the 
filter  has  been  in  service.  When  the  filter' will  not 
deliver  the  requisite  amount  of  water,  with  the  maxi- 
mum loss  of  head  allowed,  the  filters  must  be  cleaned. 
In  most  of  the  European  plants  the  loss  of  head  is 
limited  to  from  24  to  36  inches,  but  in  some  cases 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      l6l 

these  limits  are  exceeded.  There  is  no  reason  why 
the  loss  of  head  should  not  be  as  great  as  the  depth  of 
water  over  the  filters,  or  say  from  5  to  6  feet  in  ordi- 
nary cases.  When  the  loss  of  head  is  greater  than 
the  depth  of  water  over  the  sand,  clogging  may  oc- 
cur just  below  the  surface  of  the  sand,  on  account  of 
the  accumulation  of  matter  on  the  surface  and  the 
liberation  of  bubbles  of  air  from  the  water.  If  the 
regulation  of  the  rate  of  filtration  were  done  by 
throttling  the  underdrain  before  discharging  the  wa- 
ter into  the  regulating  chamber,  negative  heads  could 
be  used  and  greater  periods  of  time  between  scraping 
would  be  the  result.  In  other  words,  the  section  of 
greatest  resistance  should  be  transferred  from  the 
surface  of  the  sand  to  the  outlet  of  the  underdrains, 
if  negative  heads  are  to  be  used  successfully.  This 
occurs  with  some  forms  of  automatic  regulating  ap- 
paratus and  hence,  with  such,  negative  heads  may  be 
employed,  at  least  up  to  the  limit  of  economical  con- 
struction. 

Regulating  apparatuses  are  of  two  kinds:  those 
operated  entirely  by  hand,  and  those  which  are  auto- 
matic in  their  operation.  Hand-regulators  were  used 
in  England  as  early  as  1839  on  the  filters  built  by 
James  Simpson  for  the  Chelsea  Water  Company  at 
London.  The  apparatus  consisted  merely  of  a  valve 
in  the  supply-pipe,  and  one  in  the  discharge-pipe 
from  the  underdrains.  A  similar  arrangement  was 
used  at  the  Stralau  works  at  Berlin,  and  is  in  use  at 
Edinburgh,  Scotland.  In  the  latter  place  a  weir  was 
added  for  gauging  the  quantity  of  flow.  The  outlet 


162 


WATER  FILTRATION   WORKS. 


from  the  filters  at  Shanghai  is  also  a  simple  sluice- 
valve,  but  an  automatic  double-seated  balanced  valve 
on  the  feed-pipe,  operated  by  a  float,  keeps  the  water 
level  on  the  filters  at  a  constant  height.  It  is  evident 
that  this  construction  was  intended  to  make  the  fil- 
tration head  correspond  to  the  fluctuating  draft 
rather  than  to  regulate  the  flow  to  a  constant  rate. 
Similar  arrangements  are  found  in  most  of  the  early 
filters  and  in  many  still  in  use. 

Another  form  of  regulator  much  used  in  the  Eng- 


FIG.  25.— REGULATING  APPARATUS  IN  USE  AT  YOKOHAMA,  JAPAN. 

lish  practice  is  a  telescopic  tube,  the  upper  section 
of  which  can  be  raised  or  lowered  by  a  screw.  This 
form  of  regulator  is  in  use  at  the  New  River  Com- 
pany's filters  at  London,  at  the  Yokohama  water- 
works in  Japan  and  at  Koenigsberg,  in  Germany 
(Figs.  25  and  26).  In  this  device  a  constant  dis- 
charge, and,  therefore,  a  constant  velocity  of  nitration, 
is  insured  by  so  regulating  the  height  of  the  top  of  the 
telescopic  pipe  that  a  constant  depth  of  water  flows 
over  its  edge.  This  requires  the  screwing  down  of  the 
top  of  the  pipe,  as  the  resistances  to  filtration  become 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      163 

greater  with  the  clogging  of  the  filter.  A  modified 
form  of  the  apparatus,  designed  by  the  Stanwix 
Engineering  Company,  in  1893,  *s  m  use  at  IHon, 
N.  Y.  It  consists  of  a  telescopic  tube  13  inches  in 
diameter,  inclosed  in  a  tube  20  inches  in  diameter, 
the  smaller  tube  being  movable  and  connecting 


BEtL  MOUTH 


FIG.  26. — REGULATOR  IN  USE  AT  KOENIGSBERG,  GERMANY. 

through  a  fixed  diaphragm  in  the  2O-inch  pipe  with 
the  pipe  leading  to  the  filtered-water  reservoir. 

At  Antwerp  the  water  from  the  filter  underdrains 
comes  out  at  the  tops  of  telescopic  tubes  and  falls  on 
spreaders  to  promote  aeration  after  having  absorbed 
iron  in  the  filter-beds.  A  similar  arrangement  has 
also  been  used  quite  extensively  in  Japan  by  Pro- 
fessor Burton.  The  usual  methods  of  rating  the  dis- 
charge of  the  telescopic  tube  are  by  measurement  of 
the  amount  of  water  that  it  discharges,  with  different 
depths  of  water  over  the  lip  of  the  pipe,  by  observing 
the  actual  quantity  by  measurement,  or  by  weir 
gauging. 


164 


WATER  FILTRATION    WORKS. 


In  1884  Mr.  Henry  C.  Gill  designed  the  regulating 
apparatus  for  the  Lake  Tegel  filters  at  Berlin.  This 
apparatus  is  still  in  use  there  and  also  at  the  new 
Lake  Mueggle  works.  It  is  shown  in  Fig.  27.  There 


FIG.  27. — REGULATOR   DESIGNED  BY  HENRY  C.  GILL  AND  USED 
AT  THE  BERLIN  FILTER  PLANTS. 

are  three  chambers;  the  water  from  the  underdrains 
entering  freely  into  the  first,  then  into  the  second 
through  an  opening  controlled  by  a  valve  at  the  bot- 
tom. In  the  wall  of  the  second  chamber  a  fixed  weir 
of  known  dimensions  is  placed.  After  flowing  over 
the  weir  the  water  falls  into  the  third  chamber,  which 
connects  with  the  channel  leading  to  the  filtered 
water  reservoir.  A  constant  depth  of  water  over 
the  weir  is  secured  by  the  operation  of  the  valve  in 
the  first  chamber,  which  is  opened  or  closed  in  ac- 
cordance with  the  indications  of  floats  in  the  different 
chambers  and  on  the  water  in  the  filters.  By  the 
positions  of  these  floats  the  attendant  can  determine 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      1 65 

the  filtration  head,  rate  of  filtration  and  actual  depth 
of  water  on  the  filters,  and  will  regulate  the.  valves  ac- 
cordingly. 

The  quantity  of  water  discharged  over  the  weir 
per  second  of  time  may  be  found  by  the  formula 


Q  —  u  —  bh  \/2gh,  where  Q  =  cubic  feet  per  second, 
•j 

b  =  width  of  weir  in  feet,  h  —  depth  of  water  in  feet 
over  the  weir,  measured  back  of  the  weir  where 
the  water  is  level,  g  =  the  acceleration  of  gravity, 
=  32.2  feet  per  second,  and  u  —  a  coefficient  to  be 
experimentally  determined,  its  approximate  value 
being  about  0.60,  but  varying  between  quite  wide 
values  with  different  widths  of  weir  and  depths  of 
water  flowing  over  it. 

In  1866  Mr.  James  P.  Kirkwood  recommended  for 
St.  Louis  an  apparatusf  for  regulating  the  rate  of  fil- 
tration, which  consisted  of  a  weir  that  could  be  raised 
or  lowered  until  the  proper  quantity  would  flow  over 
it.  This  is  shown  in  Fig.  ,28.  The  regulating  appara- 
tus at  Hamburg  (Fig.  29)  is  a  modification  of  Kirk- 
wood's,  with  also  a  submerged  orifice  leading  from 
the  second  to  the  third  chambers,  through  which  the 
discharge  can  be  further  measured.  In  the  Ham- 
burg apparatus  a  scale  is  attached  to  the  movable 
weir  and  a  floating  index  on  the  surface  of  the  water 
in  the  chamber  shows  always  the  depth  of  water  run- 
ning over  the  weir;  the  scale  also  gives  the  height 
of  the  water  in  the  measuring  chamber  relative  to 
that  in  the  filters,  the  water  in  the  filters  being  kept 
at  a  constant  level  by  means  of  a  float  operating  a 


166 


WATER  FILTRATION    WORKS. 


I  FILTERED  WATER 


FIG.  28. — REGULATOR  RECOMMENDED  BY  JAS.  P.  KIRKWOOD  FOR 
ST.  Louis. 


FIG.  29. — REGULATING  APPARATUS  IN  USE  AT  HAMBURG,  GER- 
MANY.    F.  ANDREAS  MEYER,  ENGINEER. 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      1 67 

balanced  valve  on  the  supply  main.  The  discharge 
over  this  weir  and  also  over  that  of  the  Kirkwood 
apparatus  would  be  calculated  by  the  formula  just 
given  for  the  Berlin  apparatus. 

At  Albany  Mr.  Allen  Hazen  has  adopted  a  method 
of  regulation  somewhat  different  from  any  in  use  else- 
where. (Fig.  30.)  The  water  from  the  underdrains 


FIG.  30. — REGULATING  APPARATUS   DESIGNED  BY  ALLEN   HAZEN 
FOR  THE  ALBANY  FILTERS. 

enters  a  chamber,  in  one  wall  of  which  is  placed  a  thin 
plate,  with  a  long, -narrow  orifice  of  fixed  dimensions 
through  its  centre.  The  water  flows  into  the  second 
chamber  through  this  orifice;  floats  resting  on  the 
water  in  each  chamber  show  the  difference  of  level  be- 
tween the  water  surface  each  side  of  the  plate,  and 
from  this  difference  of  level  the  discharge  can  be 
computed.  Indexes  and  scales  connected  with  the 
floats  show  the  filtration  head  and  the  rate  of  filtra- 
tion. When  the  water  in  the  second  chamber  falls  be- 
low the  centre  of  the  orifice,  the  float  in  that  chamber 
is  prevented,  by  an  ingenious  arrangement,  from 


1 68  WATER  FILTRATION    WORKS. 

sinking  lower  than  that  point,  and  the  discharge 
through  the  orifice  is  then  a  free  discharge  into  the 
air.  The  size  of  the  opening  is  so  proportioned  that 
a  certain  maximum  rate  of  filtration  may  not  be  ex- 
ceeded in  service.  The  regulation  of  the  rate  of  flow 
is  effected  by  valves  worked  by  hand.  So  long  as  a 
constant  difference  of  level  is  maintained  between  the 
water  surfaces  either  side  of  the  orifice,  a  constant 
discharge  will  ensue.  With  a  little  care  the  rate  can 
be  regulated  very  closely. 

Automatic  regulators  may  be  classed  in  two 
groups:  those  operated  by  the  action  of  floats  on  the 
surface  of  the  filtered  water  and  those  in  which,  with 
varying  rates  of  draft,  the  velocity,  or  energy  of  the 
water,  as  it  is  drawn  from  the  filters,  is  made  to  effect 
its  own  regulation  by  opening  or  closing  a  balanced 
valve. 

Of  the  automatic  regulators  operated  by  floats, 
those  at  Zurich  (Fig.  31)  and  Warsaw  (Fig.  32) 
are  the  most  prominent  of  the  European  types.  In 
each  there  is  a  telescopic  joint  of  pipe,  having  ver- 
tical slits  around  the  periphery  at  the  top,  suspended 
from  a  float.  The  float  swims  on  the  surface  of  the 
filtered  water  in  the  regulating  chamber,  and  the 
filtered  water  escapes  to  the  reservoir  through  this 
telescopic  pipe.  If  the  top  of  the  pipe  is  kept  at  a 
constant  depth  below  the  surface  of  the  filtered  water 
a  constant  flow  will  be  established.  The  depth  of  im- 
mersion of  the  pipe  in  the  Zurich  apparatus  is  gov- 
erned by  a  screw  which  alters  the  relative  height 
of  the  float  and  pipe.  The  float  carrying  the  pipe  is 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      169 

free  to  move  vertically,  the  screw-stem  being  square 
above  the  float  an^  sliding  through  the  hub  of  the 


'"ft   '«». 


FIG.  31.—  REGULATOR  IN  USE  IN  ZURICH,  SWITZERLAND 
M.  PETER,  ENGINEER. 


FIG.  32. — REGULATOR  DESIGNED  BY  MR.  WM.  H.  LINDLEY  FOR 
THE  FILTERS  AT  WARSAW,  POLAND. 

gear  wheels  above.     In  the  Warsaw  apparatus,  de- 
signed by  Mr.  William  H.  Lindley,  the  relative  po- 


WATER  FILTRATION   WORKS. 

sitions  of  float  and  pipe  are  fixed  and  the  rate  of  dis- 
charge is  regulated  by  a  collar,  the  moving  of  which 
opens  or  closes  an  orifice  at  the  top.  To  Mr.  Lind- 
ley  is  due  the  credit  of  first  advocating  the  separate 
and  automatic  regulation  of  slow  sand-filters. 

A  modification  of  Mr.  Lindley's  regulator  (Fig. 
33)  was  proposed  for  the  regulation  of  the  filters  for 


FIG.  33. — TYPE  OF  REGULATOR  SUGGESTED  BY  THE  MAYOR'S 
EXPERT  WATER  COMMISSION  FOR  PHILADELPHIA,  PA. 

Philadelphia.  The  difficulty  with  these  automatic 
devices  is  that  sometimes  the  floats  are  not  given 
sufficient  margin  of  buoyancy  to  overcome  instantly 
the  friction  of  the  packing  around  the  telescopic  pipe, 
as  the  water  level  changes  in  the  regulating  chamber, 
and  this  may  affect  the  depth  of  immersion  of  the 
tube  and  consequently  the  rate  of  discharge.  For 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      I?I 

this  reason  deep,  narrow  slots  around  the  periphery 
of  the  tube  are  better  than  wide  shallow  ones,  the 
discharge  through  the  former  being  affected  in  a 
lesser  degree  for  a  given  change  of  depth  of  immer- 
sion than  through  the  latter.  With  proper  details 
of  design  and  construction  this  trouble  can  be  recti- 
fied. 

The  regulator  shown  in  Fig.  34  was  designed  by 


TO  RESERVOIR 


FIG.  34. — REGULATING  APPARATUS  DESIGNED  BY  THE  AUTHOR 
FOR  THE  TOME  INSTITUTE  FILTERS. 

the  author  for  the  Tome  Institute  filters.  The  water 
from  the  filters  discharges  into  a  well  through  which 
the  pipe  leading  to  the  filtered-water  reservoir  rises 
to  above  the  level  of  the  water  on  the  filters.  A  rect- 
angular orifice  is  cut  through  one  side  of  the  pipe  on 
line  with  its  central  horizontal  axis.  The  orifice  is 


1 72  WA  TER  FIL  TRA  TIOM   WORKS. 

formed  in  a  thin  plate  with  beveled  edges,  and  one 
side  is  movable  vertically,  like  a  slide.  This  slide  is 
attached  to  a  rod  connecting  with  the  end  of  a  lever 
operated  by  a  ball-float  and  pivoted  to  the  pipe.  As 
the  water-level  in  the  well  rises  the  slide  will  close  the 
orifice  proportionately.  The  size  of  the  orifice  is 
such  that  if  the  water-level  in  the  filtered-water  reser- 
voir were  below  the  orifice,  the  rate  of  discharge 
would  be  constant  whether  the  water  in  the  well  stood 
6.5  feet  or  .5  foot  above  the  orifice,  and  this  rate 
would  be  50  per  cent,  greater  than  the  rate  at  which 
the  filters  are  to  operate  normally.  When  the  draft 
on  the  filters  is  normal  the  orifice  is  submerged.  If 
the  draft  is  below  normal  the  filters  are  automatically 
and  slowly  shut  off. 

In  all  the  foregoing  forms  of  automatic  regulator 
the  discharge  from  the  beds  is  free,  and  the  differ- 
ence in  level  between  the  water  on  the  filters  and  in 
the  chambers  adjusts  itself  automatically  to  the  in- 
creasing resistances. 

The  apparatus  used  at  Worms  (Fig.  35),  operated 
by  a  float  on  the  surface  of  the  unfiltered  water,  re- 
quires constant  adjustment  as  the  resistances  in- 
crease, and  does  not,  it  would  seem,  offer  the  advan- 
tages given  by  the  more  simple  forms  used  at  War- 
saw and  Zurich. 

The  automatic  regulator,  designed  by  Professor 
W.  K.  Burton,  and  shown  in  Fig.  36,  utilizes  the  ve- 
locity of  the  water,  as  it  is  discharged  from  the  filter, 
to  effect  its  own  regulation.  It  is  in  use  in  the  filters 
at  Tokio  and  Osaka,  Japan,  and  consists  of  a  bal- 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      1/3 


FIG.  35.— REGULATOR  IN  USE  AT  WORMS,  GERMANY. 


FIG.  36.— REGULATOR  DESIGNED  BY  PROF.  W.  K.  BURTON  AND 

IN    USE  AT   TOKIO   AND   OSAKA,    JAPAN. 


174  WATER  FILTRATION   WORKS. 

anced  valve  opened  or  closed  by  differences  in  pres- 
sure on  the  opposite  sides  of  a  piston  attached  to  the 
valve-stem.  This  difference  of  pressure  is  induced 
by  placing  a  diaphragm,  with  an  orifice  in  its  centre, 
in  the  outlet  of  the  underdrain  pipe.  In  passing 
through  this  orifice  the  flow  of  the  water  is  retarded; 
this  produces  pressure  on  the  lower  side  of  the  pis- 
ton, as  a  consequence  of  which  the  valve  is  closed  au- 
tomatically. 

There  is  still  room  for  the  exercise  of  inventive  ge- 
nius in  the  improvement  of  the  regulating  apparatus 
for  slow  sand-filters.  Some  reliable  arrangement 
which,  while  preventing  a  certain  maximum  rate 
being  exceeded,  would  automatically  permit  the  use 
of  any  rate  below  the  maximum,  at  the  same  time 
giving  a  continuous  record  of  the  actual  rate,  would 
be  very  useful.  Such  an  apparatus  would  allow  the 
filters  to  adjust  themselves  to  some  extent  to  the 
rate  of  draft.  As  has  already  been  explained,  this 
practice  has  been  found  to  be  safe,  between  certain 
limits,  and  gives  to  the  plant  a  flexibility  in  opera- 
tion that  at  times  may  be  very  desirable. 

Cost  of  Slow  Sand-filters. — The  cost  of  construc- 
ing  slow  sand-filter  plants  depends  entirely  upon  local 
conditions.  It  is  convenient  to  refer  the  cost  to  a 
unit  of  area  rather  than  to  a  unit  of  quantity  of  water 
filtered,  because  some  waters  may  be  filtered  more 
rapidly  than  others,  and  hence  the  cost  per  million 
gallons  filtered  would  not  be  a  satisfactory  unit  for 
comparison.  The  cost  per  acre  of  filter  surface,  how- 
ever, is  a  very  convenient  and  expressive  unit,  as 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.      175 

from  it  an  idea  may  be  mentally  and  rapidly  formed 
of  the  cost  of  a  proposed  plant.  Small  filter-beds 
naturally  cost  more  per  acre  than  large  ones,  be- 
cause, while  the  cost  of  floors,  roofs  and  piers  will  be 
about  the  same  per  acre  of  area,  the  volume  of  ma- 
sonry in  the  side-walls  of  small  filters  will  be  much 
greater  in  proportion  to  the  area  than  in  the  large 
ones. 

Among  the  features  that  influence  the  cost  of  filter 
plants  are  the  rate  at  which  they  are  to  be  operated, 
the  arrangement  of  the  beds,  the  topography  and  ge- 
ological structure  of  the  region,  the  local  prices  of 
materials  and  labor,  the  legal  length  of  a  day,  the  ex- 
penses of  administration,  the  season  of  the  year  in 
which  the  work  is  undertaken,  the  rate  at  which  the 
work  will  have  to  be  pushed  and  the  difficulties  in  the 
transportation  of  materials  and  the  securing  of  labor. 

It  is  generally  found  that  the  filters  should  be  so 
placed  that  the  water  level,  when  the  filters  are  filled, 
will  be  about  at  the  level  of  the  natural  surface  of  the 
ground.  It  also  seems  to  make  little  difference  in  the 
cost  of  excavation  whether  the  filters  are  built  on 
level  ground  or  on  sloping  ground,  with  the  beds 
stepped  down  in  terraces,  even  when  there  is  con- 
siderable difference  in  elevation  between  different 
parts  of  the  area.  For  instance,  in  the  data  given 
in  Table  XII,  the  differences  in  elevation  of  the  nat- 
ural surface  of  the  ground  in  different  parts  of  plants 
Nos.  i  and  2  was  fifteen  feet;  in  plants  Nos.  3  and  4, 
ten  feet;  in  plant  No.  5,  thirty  feet,  and  in  plants  Nos. 
6  and  7,  five  feet.  In  plant  No,  8  the  excavation  was 


WATER  FILTRATION    WORKS. 

nearly  all  slate  rock.  In  plants  Nos.  9  and  10  there 
was  also  a  large  amount  of  rock  excavation. 

Data,  concerning  the  cost  of  filter  plants,  derived 
from  works  in  operation,  must  be  interpreted  with  a 
thorough  understanding  of  what  these  costs  include, 
and  a  knowledge  of  the  local  conditions.  Some 
works,  very  simple  in  design,  requiring  no  pumping 
machinery,  no  difficult  foundations  and  no  excessive 
haul  on  materials,  can  be  built  cheaply,  while  others, 
where  the  conditions  are  not  so  favorable,  may  cost 
much  more.  In  the  estimates  of  cost  of  the  filter 
plants  for  the  improvement  of  the  Philadelphia 
water-supply,  the  transporting  of  the  materials  to  the 
filter  siteswas  an  item  of  considerable  magnitude.  Its 
effect,  exclusive  of  the  hauling  of  the  piping,  was  to 
increase  the  cost  of  the  remote  plants  at  the  rate  of 
about  $4,500  per  acre  above  the  cost  of  the  plants 
more  favorably  located;  the  beds  were  covered  and 
had  an  area  of  f  acre  each.  It  is  necessary  therefore 
when  the  plant  is  to  be  built  at  a  point  where  consid- 
erable difficulties  attend  the  delivery  of  materials,  to 
add  to  their  cost  the  expense  of  transporting  them  to 
the  site  of  the  works. 

In  Table  XII  are  classified  the  unit  costs  per  acre 
of  some  of  the  component  parts  of  several  large  filter 
plants  in  the  United  States  for  which  estimates  of 
cost  have  recently  been  made.  All  the  filters  were 
to  have  been  covered  and  to  have  an  area  of  about 
f  acre  each.  The  estimates  do  not  include  the  cost  of 
the  piping  to  and  from  the  filters,  the  cost  of 
pumping  plants,  of  sedimentation  basins  or  filtered- 


CONSTRUCTION  OF  SLOW  SAND-FILTERS.       1 77 


water  reservoirs,  but  do  include  the  making  of  roads, 
sodding  of  embankments,  seeding  of  lawns,  and  all 
work  connected  with  the  filters,  including  the  un- 
derdrains,  filtering  materials,  regulating  apparatus, 

etc. 

TABLE  XII. 


, 

Number 
of  Beds. 

Excava- 
tion, 
including 
Sodding, 
Seeding, 
etc. 

Sand- 
washers, 
including 
Piping  for 
Wash- 
water. 

Covered 
Filters 
Comolete, 
including 
Filtering 
Materials, 
etc. 

Electric- 
lighting 
Plant. 

Tramways 
for  Sand- 
Hauling, 
including 
Cars. 

Resi- 
dences, 
Shelters, 
Offices, 
Fences, 
Store- 
rooms, etc. 

I 

2 

13 
26 

g 

$3,200 
3,200 
3  208 

$328 
276 
768 

$50,313 
44,348 
50  616 

$640 
772 

$536 
572 
468 

$3600 
2308 

C  flf\ 

18 

2  852 

o-ift 

CQ  J.I2 

A  A  A 

2O68 

5 
6 

27 

24 

3,276 
3O76 

276 

•7  12 

49,808 
50  228 

740 

588 

e  AQ 

1904 
21^6 

n8 

3OJ.O 

•^6 

49  808 

c28 

T  AA.R 

8 
9 

10 

136 
133 
70 

24,716 

8,000 
16,048 

412 

280 
290 

50,000 
50,000 
46,668 

I5"7O 
1  200 
1240 

6.6 

452 
536 

1508 
1416 
1500 

The  Albany  covered  filters  cost  about  $38,000  per 
acre,  including  the  filtering  materials,  but  excluding 
the  excavation,  sand-washing  machinery,  buildings, 
pumps,  settling  basins,  and  piping  to  and  from  the 
filters,  and  about  $45,600  per  acre,  including  all  the 
above  items,  except  the  pumps  and  sedimentation 
reservoirs.  Small  plants  cost  very  much  more  in 
proportion  than  large  ones.  For  instance,  for  a  plant 
consisting  of  three  open  filters,  with  a  total  area  of 
0.19  acre,  the  actual  costs  per  acre  were  as  follows: 

Excavation,  grading,  etc $8,200 

Sand-washing  machinery 8,400 

Filter-beds,  including  sand,  etc 100,000 

Tramways  and  equipment 816 


178  WATER  FILTRATION   WORKS. 

For  another  plant  consisting  of  two  very  small  cov- 
ered filters  with  a  total  area  of  0.013  acre>  the  costs 
per  acre  were  as  follows: 

Excavation,  grading,  etc $17,000 

Covered  filters,  including  sand,  etc. .  . .    115,400 

The  cost  of  the  Nyack  filters  with  a  total  area  of 
0.38  acre  was,  for  excavation,  including  foundations, 
sheet  piling,  etc.,  $30,500  per  acre,  and  for  the  open 
filters  complete  $46,700  per  acre.  The  covered  slow 
sand-filters  at  Ashland,  Wis.,  with  an  area  of  half  an 
acre,  cost  at  the  rate  of  a  little  under  $70,000  per 
acre. 

Statements  of  cost  per  acre  must  therefore  be  in- 
terpreted understandingly.  The  necessary  piping, 
the.  drains,  auxiliary  pumping  machinery  for  lifting 
the  water  to  the  filters  from  the  settling  basins,  or 
into  the  settling  basins  from  the  source  of  supply, 
the  land,  buildings  and  other  necessary  adjuncts  may 
amount  to  nearly  as  much  as  the  cost  of  the  filters. 
These  conditions  are  so  varying  that  statements  of 
their  cost  in  individual  cases  would  be  of  little  value 
here.  The  cost  of  the  bacteriological  and  chemical 
laboratories  cannot  well  be  stated  in  a  price  per  acre, 
because  one  laboratory  generally  serves  an  entire 
municipal  plant,  and  requires  about  the  same  equip- 
ment for  a  small  plant  as  for  a  large  one.  The  cost 
of  such  a  laboratory,  properly  equipped,  is  about 
$30,000,  but  may  be  more  or  less  than  this  by  a  con- 
siderable amount,  according  to  circumstances. 

The  cost  of  roofing  the  Albany  filters,  including 


OPERATION  OF  SLOW  SAND-FILTERS. 

the  piers,  was  about  $0.315  per  square  foot,  or  a  little 
under  $14,000  per  acre.  In  the  estimates  given  in 
the  preceding  tabulation  the  cost  of  the  roofing  was, 
in  most  cases,  very  close  to  this  figure. 

OPERATION. 

For  convenience  in  operating  the  plant  it  will  be 
advantageous  to  place  the  filtered-water  reservoir 
at  such  a  height  that  the  filtered  water  may  be  con- 
ducted to  it  by  gravity.  The  highest  water  level  in 
the  reservoir  should  be  such  that  it  will  not  cause 
back  water  on  the  filters  and  thus  limit  the  filtration 
head  in  the  different  beds  when  the  plant  is  operat- 
ing at  its  maximum  capacity.  Such  an  arrangement 
will  permit  the  filters  to  operate  independently  when 
the  draft  is  normal,  while  at  the  same  time  it  will 
cause  the  water  level  in  the  reservoir  and  regulating 
chambers  to  rise  when  the  draft  falls  below  normal, 
slowly  and  automatically  reducing  the  filtration  head 
on  the  filters  and  affecting  those  first  which  have 
been  longest  in  service.  Upon  the  draft  again  being 
increased  the  water  level  in  the  reservoir  will  fall  and 
the  rate  of  filtration  in  the  different  beds  will  grad- 
ually be  increased  to  the  rates  at  which  they  were  last 
operating.  This  principle  is  applicable  to  both  the 
automatic  regulators  and  the  submerged-orifice  ap- 
paratus designed  for  the  Albany  filters  by  Mr.  Allen 
Hazen.  The  effect  of  the  gradual  changing  of  the 
rate  of  filtration  between  reasonable  limits  has  al- 
ready been  shown  to  have  no  bad  effects  on  the  qual- 


180  WATER  FILTRATION    WORKS. 

ity  of  the  effluent,  while  the  adoption  of  such  a  plan 
offers  many  advantages. 

Scraping  Slozv  Sand-filter  Beds. — After  a  filter-bed 
has  been  in  operation  for  a  considerable  time  it  be- 
comes so  clogged  at  the  surface  that  the  water  can- 
not pass  through  it  at  the  prescribed  rate.  When 
this  time  comes  it  is  necessary  to  put  out  of  service 
and  clean  the  bed.  The  first  operation  will  be  to  drain 
off  the  raw  water  standing  above  the  sand,  and  lower 
its  level  below  the  surface  of  the  filter  so  that  the 
workmen  may  enter  after  the  sand  is  hard  enough  to 
bear  their  weight.  Cleaning  is  now  done  by  hand, 
although  undoubtedly  improvements  in  methods 
will  be  brought  out  as  filter  plants  multiply.  The 
workmen  are  furnished  with  broad  flat  shovels  or 
scrapers  with  which  they  skim  off  the  dirty  top  layer 
of  the  sand  to  sufficient  depth  to  remove  the  clog- 
ging— from  f  inch  to  a  little  over  an  inch,  generally, 
averaging  perhaps  somewhat  less  than  f  inch.  The 
method  is  illustrated  in  Plate  XL  This  material  is 
heaped  up  in  piles  on  the  surface  of  the  filter  and  then 
removed  in  wheelbarrows  on  plank  runways,  or  in 
cars  running  on  movable  tracks.  In  some  cases  the 
dirty  sand  is  lifted  out  of  the  manholes  of  covered 
filters  by  derricks.  Wheelbarrows  are  generally  used 
in  small  plants,  and  in  large  ones  wheelbarrows  to 
get  the  sand  to  the  tramway,  and  cars  from  there  to 
the  sand-washers.  The  sand,  as  it  is  removed,  is  taken 
to  the  court  and  deposited  in  piles  near  the  sand- 
washers.  The  washing  is  done  only  in  warm  weather, 
the  winter's  accumulations  being  allowed  to  stand 


OPERATION  OF  SLOW  SAND-FILTERS.         183 

over  till  spring.  Storage-room  for  this  material  must 
therefore  be  provided,  as  washing  can  not  be  prop- 
erly done  in  freezing  weather. 

The  cost  of  the  labor  of  scraping  filters  varies  con- 
siderably in  different  plants  and  in  different  coun- 
tries. 

Mr.  George  I.  Bailey  gives  the  following  data*  for 
the  Albany  plant: 

Average  depth  of  sand  removed  at  each  scraping I  in. 

Hours  of  labor  to  scrape  I  acre 67 

Wheeling  out  scraped  sand;  average  haul  going  and  coming, 

600  feet;  speed  per  man  per  hour 1.18  miles 

Quantity  of  sand  wheeled  out  per  hour's  work 0.38  cu.  yd. 

Quantity  of  sand  washed  per  hour's  work. 0.41  cu.  yd. 

Quantity  of  water  used  for  washing  sand 12-14  volumes 

Quantity  of  sand  replaced  per  hour's  work 0.52  cu.  yd. 

Quantity  of  water  filtered  between  scrapings,  66,600,000  gallons 

per  acre  of  filter  surface. 

The  refilling  was  done  mostly  by  extra  labor. 

Cost  of  Scraping. — The  time  required  for  scraping 
an  acre  of  filter  surface  ranges  from  65  hours  to 
about  300  hours  in  the  different  plants  from  which 
the, author  has  been  able  to  obtain  data;  a  fair  and 
ordinarily  attainable  result  with  covered  filters 
would  be  about  175  hours  per  acre.  The  annual 
deep  scraping  requires  much  more  time  than  this  and 
may  be  estimated  by  the  cubic  yard  when  the  quan- 
tity to  be  removed  is  known.  Ordinarily,  at  such 
times,  the  sand  will  have  to  be  taken  out  for  a  depth 
of  from  4  to  8  inches,  but  in  some  cases  it  may  be 
necessary  to  remove  it  all  down  to  the  surface  of  the 


Trans.  Am.  Soc.  C.  E.,  vol.  XLIII.  p.  296. 


184  WATER  FILTRATION   WORKS. 

gravel.  At  Lawrence,  Mass.,  in  1898,  the  filters  be- 
came partially  clogged,  so  that  their  capacity  was 
considerably  reduced.  An  investigation  by  the  State 
Board  of  Health  revealed  in  the  gravel  a  growth  of 
crenothrix,  which  had  caused  a  deposit  of  iron-rust 
to  such  an  extent  around  and  between  the  stones 
that  the  water  could  not  pass  freely  into  the  under- 
drains.  The  growth  of  crenothrix  was  found  to  be 
due  to  the  pumping  of  the  water  away  from  the  fil- 
ters too  rapidly,  thus  unduly  lowering  its  level  and 
permitting  air  to  enter  the  underdrains.  The  trouble 
was  rectified  by  excavating  a  large  part  of  the  area 
and  renewing  the  underdrainage  system. 

After  the  annual  deep  scraping  it  is  customary  to 
loosen  up  the  remaining  sand  for  a  depth  of  several 
inches  and  allow  the  filter  to  stand  for  some  time, 
several  days  in  some  instances,  before  refilling  with 
washed  sand.  The  surface,  after  scraping,  is  raked 
over  to  make  it  level  and  smooth  and  to  remove  the 
prints  of  the  workmen's  boots.  In  some  places,  par- 
ticularly in  England,  it  is  customary,  once  a  year,  to 
trench  the  sand  down  to  the  gravel,  filling  the 
trenches  with  washed  sand,  and  afterward  covering  this 
with  the  sand  taken  from  the  trenches.  Experiments 
have  also  been  made  with  the  "seeding"  of  the  beds 
after  scraping  by  spreading  a  thin  layer  of  partially 
clogged  sand  over  the  filters  to  start  the  biological 
action  more  quickly,  but  so  far  as  I  have  been  able 
to  learn  the  process  has  not  proven  of  any  advantage. 

The  quantity  of  sand  removed  at  a  scraping,  as- 
suming the  layer  taken  off  to  be  f  inch  deep,  would 


OPERATION  OF  SLOW  SAND-FILTERS. 


be  about  50^  cubic  yards  per  acre.  If  it  were  nec- 
essary to  scrape  13  times  during  the  year,  includ- 
ing the  annual  deep  scraping,  and  if  the  latter  were 
4  inches  deep,  the  quantity  of  sand  removed,  per 
acre,  would  be  about  1,150  cubic  yards,  equivalent 
to  an  average  .depth  of  8^  inches.  At  Lawrence 
the  quantity  has  been  slightly  less  than  this. 

Frequency  of  Scraping. — The  frequency  with  which 
scraping  will  be  required  depends  principally  on  the 
character  of  the  water,  being  necessary  more  fre- 
quently at  some  seasons  of  the  year  than  at  others. 
The  following  table,  compiled  from  the  reports  of 
the  Lawrence  Water  Board  for  1897  and  1898,  show 
the  number  of  times  each  of  the  filter-beds  was 
scraped  during  these  two  years. 

TABLE   XIII. 


Year. 

1897 

1898 

Month. 

Number  of  Filter-bed. 

i 

2 

3 

4 

5 

6 

7 

8 

9 

10 

II 

12 

13 

M 

15 

16 

*7 

18 

19 

20 

•i  I 

•22 

23  24 

Jan. 
Feb. 

March 

May 
June 
July 
Au£. 
Sept. 
Oct. 
Nov. 
Dec. 
[an. 
Feb. 
March 
April 
May 
June 
July 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 

i 

I 

i 

i 

i 

X 

r 

X 

X 

I 

i 
i 

X 

I 

i 
i 

i 

i 
r 

X 

i 

j 

1 

X 

X 

I 

I 

X 

I 

I 

i 

I 

I 

i 

i 

I 

\ 

r 

X 

X 

I 

j 

j 

I 

I 

I 
I 

I 
I 

1 

I 

I 

I 

I 

X 

I 

I 

X 

i 

I 

i 

1 

1 

i 

X 

X 

I 

I 
I 

I 

I 

' 

• 

I 

I 

I 
I 

X 
X 

I 

x 

i 

I 

\ 

i 

i 

X 

\ 

! 

i 

i 
i 

I 
I 

\ 

i 

i 
I 

X 
X 

i 
i 

i 

i 

I 

' 

X 

X 

J 

I 

I 

I 

I 

1 

I 

1 

X 
X 

I 
I 

1 

X 

1 

X 

I 

I 

X 

1 
I 

I 

I 

I 

I 

I 

i 

i 

I 

I 

i 

- 

I 

I 

' 

: 

I 

1 

I 

I 

I 

I 

1 86 


WATER  FILTRATION  WORKS. 


The  average  number  of  scrapings  at  Lawrence  has 
been  about  14  per  year. 

In  Zurich,  notwithstanding  the  clearness  of  the 
lake  water,  the  filters  require  scraping  quite  fre- 
quently at  times,  on  account  of  the  presence  of  cer- 
tain organisms  in  the  water  in  the  summer  season 
which  clog  the  surface  of  the  sand  very  rapidly.  The 
following  tabulation  exhibits  the  data  regarding  the 
scraping  of  the  Zurich  filters  for  several  years. 


TABLE   XIV. 


Year.... 

3,895 

1896 

1897 

Days  between  Scrapings. 

7  Filters. 

7  Filters. 

10  Filters. 

10  Filters. 

e 

5* 

5 

47 

28 

42 

64 

21 

I31 

17 

17 

Average,  number  of  scrap- 
ings per  year  of  each  bed. 

2<J 

18 

21 

At  the  Lake  Tegel  works  in  Berlin  the  minimum 
period  of  time  between  scrapings,  that  is,  when  algae 
growths  are  most  flourishing,  is  about  10  days;  the 
maximum  period  is  about  80  days,  occurring  in  the 
winter  time,  and  the  average  period  30  days.  At 
Altona  the  minimum  period  is  10  and  the  maximum 
50  days. 

Where  scraping  would  be  required  oftener,  on  the 
average,  than  twice  a  month,  it  may  generally  be  as- 
sumed that  some  preliminary  treatment  before 
filtration  would  be  advisable.  This  may  be  sedimen- 
tation, with  or  without  coagulation,  or  some  other 


OPERATION  OF  SLOW  SAND-FILTERS.      187 

measure  for  the  removal  of  the  suspended  matter  or 
growths  of  algae  in  the  water. 

In  existing  plants  the  quantity  of  water  filtered 
between  scrapings,  where  the  water  has  had  proper 
treatment  before  being  admitted  to  the  filters, 
ranges  from  about  40  to  100  million  gallons  per  acre. 
At  Stralau,  Berlin,  in  1893,  the  quantity  delivered 
by  the  open  filters  between  scrapings  during  the  pe- 
riod when  algae  growths  were  most  flourishing  was 
on  one  occasion  reduced  to  only  14  million  gallons 
per  acre. 

Effect  of  Covers  on  Frequency  of  Scraping. — In 
Zurich  it  was  found  that  the  open  filters  required 
scraping  more  often  than  those  which  were  covered. 
In  1887  the  average  period  between  cleanings  of  the 
covered  filters  was  77  days,  while  the  uncovered 
filters  required  cleaning  on  the  average  every  48 
days.  In  1892  and  1893  both  kinds  of  filters  were 
cleaned  more  frequently  than  in  1887,  yet  the  open 
filters  required  the  treatment  at  shorter  intervals 
than  the  covered  ones,  as  is  shown  by  the  following 
tables  taken  from  the  Stadtrat  Report  of  1893: 

TABLE   XV. 

NUMBER    OF    DAYS    BETWEEN    CLEANINGS. 


Coverec 

Filters. 

Open 

Filters. 

1892. 

1893. 

1892. 

1893.* 

IQ 

12 

II 

13 

6Q 

T\ 

CO 

•7Q 

•*6 

27 

2T 

2O 

*  1893  to  the  middle  of  September. 


WATER  FILTRATION  WORKS. 


TABLE  XVI. 

GALLONS    OF   WATER    FILTERED   BETWEEN   CLEANINGS. 


Covered  Filters. 

Open  Filters. 

1892. 

1893. 

1892. 

1893. 

Minimum..  . 
Maximum  .  . 

26,420,000 
76,618,000 

9,168,000 
116,658,000 

19,419,000 
47,952,000 

13,606,000 
51,572,000 

Average.  . 

44,914,000 

31,255,000 

29,855,000 

21,532,000 

A  thick  layer  of  green  algae  would  frequently  grow 
upon  each  of  the  uncovered  niters.  This  growth  not 
only  rapidly  clogged  the  filters,  but  also  gave  much 
trouble  in  other  directions.  The  rising  of  bubbles 
of  gas  through  the  water,  tearing  loose  and  carrying 
with  them  patches  of  the  algae  growths,  would  disturb 
the  surface  of  the  niters  and  allow  raw  water  to 
pass  through  the  bald  patches  too  rapidly.  The  cov- 
ered niters  were  not  troubled  in  this  way,  because  the 
algae  could  not  grow  in  the  dark. 

In  regard  to  the  growth  of  algae  on  filter  surfaces, 
Mr.  Charles  E.  Fowler,  Superintendent  and  Engi- 
neer of  Public  Works,  Poughkeepsie,  N.  Y.,  says:  * 
"  The  algae  growths  on  the  sand  in  summer  are  quite 
as  troublesome  and  almost  as  expensive  as  ice  and 
frost  in  winter.  Like  ice,  they  can  develop  on  an 
unlimited  area  in  the  same  time  as  on  a  small  unit, 
and  will  stop  a  filter  and  put  it  out  of  service  just 
when  it  should  otherwise  be  doing  its  best  work." 

The  studies  of  Dr.  Otto  Strohmeyer  of  the  growths 
of  microscopic  organisms  in  the  sand  of  the  Ham- 
burg filters,  and  of  Dr.  Ad.  Kemna,  of  Antwerp, 

*  Trans.  Am.  Soc.  C.  E.,  vol.  XLIII.  p.  311. 


OPERATION  OF  SLOW  SAND-FILTERS.         \§§ 

along  similar  lines,  have  brought  out,  among  other 
things,  the  very  interesting  facts  that  when  the  vege- 
tation over  the  sand  surface  is  in  a  living  condition 
it  is  a  decided  aid  to  the  efficiency  of  the  process  of 
filtration,  if  it  does  not  result  in  a  disturbance  of  the 
sand  surface,  and  that  some  of  the  algae  exercise  a 
sterilizing  influence  on  the  water  in  which  they  are 
growing;  also  that  the  flora  change  with  the  sea- 
sons, and  that  the  decomposition  of  certain  of  the 
organisms,  with  seasonal  changes,  notably  Ana- 
boena,  causes  a  bad  taste  in  the  water.  Mr.  George 
C.  Whipple,  Director  of  the  Mount  Prospect  Labora- 
tory of  the  Brooklyn  Water-works,  has  also  noticed 
that  when  the  growth  of  some  of  these  organisms, 
particularly  Asterionella  and  Synedra,  was  luxuriant 
in  the  Brooklyn  reservoirs  the  number  of  water  bac- 
teria was  unusually  low.  It  may  thus  be  that  some  of 
these  growths  exercise  a  sterilizing  influence  on  the 
water,  and,  therefore,  assist  in  its  purification,  but  it 
is  always  at  increased  cost  of  operation  of  the  filters 
on  account  of  the  more  rapid  surface  clogging. 

At  Antwerp  the  algae  growths  are  watched  care- 
fully, and  the  filters  are  operated  slowly  during  the 
season  when  such  growths  are  most  vigorous,  be- 
cause the  evolution  of  gases  breaks  loose  -large 
masses  of  the  organisms,  which,  floating  to  the  sur- 
face, carry  with  them  parts  of  the  surface  film,  and 
leave  bare  portions  of  the  unclogged  sand,  through 
which  the  water  may  pass,  imperfectly  filtered.  These 
facts  should  in  each  case  be  considered  in  deciding 
whether  or  not  covers  for  filters  are  advisable. 


WATER  FILTRATION   WORKS. 

Transportation  of  Sand  to  Wasliers. — The  cost  of 
transporting  the  sand  from  the  filters  to  the  sand- 
washers  and  back  will  depend  upon  the  distance  the 
materials  have  to  be  moved  and  upon  the  means 
employed  for  their  transportation.  Where  wheel- 
barrows are  used  the  cost  may  range  from  20  cents 
to  40  cents  per  cubic  yard,  each  way;  if  cars  are  used 
the  cost  may  be  considerably  less  than  this,  and  a 
still  further  reduction  may  be  possible  if  water-car- 
riage is  feasible. 

In  Plate  XII  is  given  a  view  of  the  Albany,  N.  Y., 
filters,  showing  the  wheeling-gang  removing  the 
scraped  sand  from  the  filters  to  the  sand-court.  This 
is  done  by  "  stint  work,"  for  which  time  and  one  half 
is  paid.  The  best  record  of  the  gang  is  as  follows: 

7.5       barrows  per  cubic  yard. 
10.5       barrow-loads  per  hour's  work. 
0.087  miles  per  barrow-load. 

The  sand-washers,  as  originally  built,  are  shown  in 
Plate  XIII.  The  dirty  sand  was  wheeled  to  the  wash- 
ers from  the  heap  in  barrows.  In  Plate  XIV  is  shown 
an  improvement,  recently  introduced,  by  which  the 
transportation  is  done  in  flowing  water  instead  of 
wheelbarrows.  A  portable  ejector  hopper  has  been 
added  to  the  washers,  so  that  the  dirty  sand  may  be 
conveyed  from  the  heap  to  the  washers  without  the 
use  of  wheelbarrows. 

Cost  of  Sand  Washing. — After  the  sand  is  taken  to 
the  ejector  washers  one  man  can  feed  it  in  as  fast 
as  two  can  take  it  away  after  it  is  washed;  the  extra 


2    1 

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OPERATION  OF  SLOW  SAND-FILTERS.         1 97 

labor  amounts  to  about  the  time  of  another  man,  or, 
say  four  men  to  wash  from  16  to  30  cubic  yards  of 
sand  per  day,  or  from  0.40  to  0.75  cubic  yard  per 
hour's  work.  Washing  the  sand  with  water  under 
pressure,  in  ejector  hoppers,  takes  from  12  to  15  vol- 
umes of  water  to  one  of  sand  washed,  or,  say  325  to 
400  cubic  feet  of  water  to  the  cubic  yard  of  sand.  This 
amounts  to  about  one  half  of  i  per  cent,  of  the  water 
filtered,  counting  the  scrapings  about  three  quarters 
of  an  inch  deep  and  the  quantity  filtered  between 
scrapings  about  80,000,000  gallons  per  acre.  An 
allowance  of  i  per  cent,  of  the  water  filtered  for  wash- 
ing sand  and  wasting  will  generally  be  ample. 

The  Trommel  washer,  or  revolving  drum,  used  at 
Berlin,  is  1 1  feet  long,  about  4  feet  in  diameter,  and 
when  turning  at  the  rate  of  7  revolutions  per  minute 
will  wash  4  cubic  yards  per  hour;  4  to  5  men  are 
required  for  operating  it,  and  about  350  to  390  cubic 
feet  of  water  are  required  for  properly  washing  a 
cubic  yard  of  sand.  The  cost  of  washing  the  sand  is 
given  as  31^  cents  per  cubic  yard,  including  the  de- 
livery to  and  removal  from  the  washer. 

The  washing  of  the  sand  is  generally  done  with 
filtered  water  from  the  mains,  but  this  is  not  abso- 
lutely necessary,  as  ordinarily  the  raw  water,  unless 
exceedingly  polluted,  will  give  satisfactory  results. 

If  the  sand  is  very  fine,  or  can  be  had  very  cheaply, 
it  may  not,  under  some  circumstances,  pay  to  wash 
the  sand  removed  during  the  periodical  scrapings. 
In  such  cases  new  sand  is  used  in  refilling. 


198  WATER  FILTRATION    WORKS. 

Lost  Sand. — In  washing  the  sand  a  certain  amount, 
depending  upon  the  uniformity  coefficient,  is  lost  by 
being  carried  away  in  the  wash-water.  Reliable  data 
bearing  on  this  subject  are  difficult  to  obtain,  but 
such  statements  as  have  come  under  the  author's  ob- 
servation lead  to  the  belief  that  from  about  3  to  10 
per  cent,  of  the  sand  washed  is  lost  in  this  way.  These 
figures  may  be  high  for  plants  using  coarse,  uniform 
sands,  but  they  are  certainly  not  high  for  those  using 
very  fine  sands.  The  amount  lost  may  be  controlled, 
to  a  certain  extent,  by  carefully  regulating  the  quan- 
tity of  sand  fed  to  the  hoppers,  the  quantity  of  water 
used  in  washing  it,  and  the  pressure  of  the  water. 
The  effect  of  repeated  washings  is  to  slightly  increase 
the  effective  size  of  the  sand  and  reduce  its  uniformity 
coefficient. 

Ice  on  Open  Filters. — Where  ice  has  formed  over  the 
water  on  open  filters  the  general  custom  is  to  remove 
it  before  scraping,  but  at  Hamburg  the  cleaning  is 
done  with  a  Mager  scraper,  a  bag  having  a  sharp  lip 
across  the  edge  of  the  open  end.  This  bag  is  sus- 
pended from  a  float  and  is  dragged  back  and  forth 
across  the  filter,  by  means  of  ropes,  allowing  the 
water  and  ice  to  remain  on  the  filters.  The  process 
is  reported  to  be  satisfactory.  The  removal  of  the 
ice  from  such  large  beds  as  those  at  Hamburg  would 
be  attended  with  much  inconvenience,  not  only  in  the 
handling  of  the  ice,  but  in  finding  a  place  to  store  it 
on  the  banks. 

The  scraping  of  open  filters  in  freezing  weather  is 
generally  very  unsatisfactory  from  all  points  of  view. 


OPERATION  OF  SLOW  SAND-FILTERS.         1 99 

The  freezing  of  the  surface  of  the  sand  makes  it  im- 
possible to  remove  a  layer  of  the  same  thickness  over 
the  whole  area,  and  also  may  cause  a  very  considera- 
ble reduction  in  the  efficiency  of  filtration  by  the  frost 
extending  down  into  the  bed  several  inches  and  form- 
ing cracks  through  which  the  water  may  start  to  fil- 
ter so  rapidly  as  to  wash  the  clogging  matter  out  and 
leave  spots  of  less  age  biologically  than  the  main 
body  of  the  filter.  The  difficulty  from  freezing,  how- 
ever, does  not  follow  until  the  temperature  is  several 
degrees  below  the  freezing  point. 

Covers  for  filters  may  also  conduce  to  economy  in 
operation  in  point  of  the  amount  of  sand  removed  in 
cleaning  during  the  summer  time  as  well  as  during 
the  winter.  In  the  open  type,  on  account  of  the  bak- 
ing of  the  surface  of  the  filter  in  the  summer  under 
the  action  of  the  hot  sun,  it  is  often  impossible  to  re- 
move as  thin  a  layer  of  sand  in  scraping  as  could  be 
easily  taken  off  if  the  baking  were  prevented. 

Refilling  after  Scraping. — After  the  filters  have 
been  scraped  they  are  refilled  with  filtered  water 
through  the  underdrains,  the  water  passing  upward 
through  the  filtering  material  until  it  stands  a  few 
inches  above  the  sand.  The  filter  is  then  allowed  to 
stand  for  a  longer  or  shorter  time  before  again  being 
placed  in  operation.  This  method  is  much  more  sat- 
isfactory than  the  older  one  of  filling  from  above  with 
raw  water.  In  the  latter  method  sub-surface  clog- 
ging and  channels,  through  which  the  water  may 
pass  freely  to  the  underdrains,  are  apt  to  be  produced 
by  the  entrainment  of  air  bubbles  and  their  rising 


200  WATER   FILTRATION    WORKS. 

through  the  filters.  It  is  also  almost  impossible  to 
pass  the  water  over  the  surface  of  a  filter  without 
washing  furrows  in  the  sand.  This  necessitates  the 
wasting  of  a  considerable  amount  of  the  water  first 
passing  through. 

Doubk  Filtration. — For  the  purpose  of  removing 
turbidity  and  to  prevent  the  clogging  of  the  filters  by 
algae  growths,  double  filtration  has  been  practised  at 
Altona,  Bremen,  Schiedam  and  Zurich.  At  Altona 
it  was  not  found  to  be  of  much  advantage,  but  at 
Bremen  it  has  proven  satisfactory.  As  generally 
carried  out,  the  method  of  operation  is  to  pass  the 
filtrate  from  a  new  filter,  or  one  which  has  recently 
been  scraped,  through  another  filter  that  has  been  in 
service  for  some  time.  With  some  waters  this  pro- 
cess will  not  prove  advantageous,  because  of  the  re- 
moval from  the  water,  by  the  first  filter,  of  the  con- 
stituents necessary  for  the  production  of  the  surface 
film. 


CHAPTER   V. 

THE   PURIFICATION   OF  WATER   BY   RAPID 
SAND-FILTRATION. 

THEORY  OF  RAPID  SAND-FILTRATION. 

The  Coagulant  and  its  Effect  on  the  Efficiency  of  Fil- 
tration.— The  discussion  which  has  preceded  has  had 
reference  only  to  what  takes  place  naturally  in  beds 
of  sand  when  water  is  passed  through  them  at  com- 
paratively slow  rates.  An  attempt  to  pass  water  very 
rapidly  through  such  beds  would,  in  a  short  time,  re- 
sult in  filling  up  the  pores  of  the  bed  and  producing 
an  effluent  no  better,  and  possibly  even  worse,  than 
the  raw  water.  If,  however,  a  coagulant  be  intro- 
duced into  the  water,  before  it  is  passed  through  the 
filters,  a  considerable  degree  of  purification  can  be 
accomplished.  Sulphate  of  alumina  has,  so  far,  been 
found  to  be  the  most  suitable  coagulant  for  the  pur- 
pose. This  compound,  when  mixed  with  water  con- 
taining a  small  amount  of  lime  or  magnesia,  breaks 
up,  forming  sulphuric  acid  and  aluminum  hydrate. 
The  sulphuric  acid  unites  with  the  alkaline  constitu- 
ents of  the  water,  while  the  hydrate  of  alumina  acts  as 
a  coagulant,  gathering  together  in  flocculent  masses 
the  particles  of  suspended  matter  in  the  water.  The 

201 


202  WATER   FILTRATION    WORKS. 

hydrate  of  alumina  is  a  sticky,  gelatinous  substance, 
which  adheres  to  the  grains  of  sand  as  the  water 
passes  through  the  filter,  and  catches  and  holds  in 
its  mass  the  bacteria,  as  well  as  the  particles  of  clay 
and  other  suspended  matter  in  the  applied  water. 
Upon  this  coagulating  material  depends  the  efficiency 
of  the  well-known  mechanical  or  rapid  sand-filters. 

As  before  stated,  the  aluminum  hydrate  forms  a 
film  of  gelatinous  or  jelly-like  material  over  the  top 
of  the  sand,  as  well  as  around  each  grain,  through 
which  the  water  must  pass  and  come  into  intimate 
contact  in  passing  down  through  the  filter.  The  sus- 
pended matter,  including  the  bacteria,  will  be  re- 
tained in  the  body  of  the  filter.  After  a  certain  period 
of  service  the  filter  will  become  clogged.  The  clean- 
ing is  done  by  agitating  the  bed  of  sand,  and  at  the 
same  time  forcing  pure  water  upward  through  it. 
The  wash-water,  containing  the  impurities  that  have 
been  retained  in  the  bed,  is  wasted,  or  turned  into 
settling  basins. 

For  a  number  of  years  experimenters  have  been 
trying  to  produce  a  cheap,  effective  coagulating  ma- 
terial. Efforts  have  also  been  made  to  cheapen  the 
present  processes  for  the  manufacture  of  sulphate  of 
alumina,  but  there  seems  to  be  no  immediate  pros- 
pect of  greatly  decreasing  its  cost  by  radical  changes 
in  methods  of  manufacture;  increased  demand,  how- 
ever, would  undoubtedly  lessen  the  price  in  the 
course  of  time. 

For  the  removal  of  turbidity  only,  the  hydrate  of 
iron  is  an  excellent  coagulant,  but  as  it  is  more  ex- 


THEORY  OF  RAPID    SAND-FILTRATION.         203 

pensive  than  alum,  and  less  efficacious  in  removing 
color,  it  is  not  used  extensively  in  connection  with 
rapid  sand-filters  in  the  United  States. 

The  only  objection  that  has  been  seriously  urged 
against  the  use  of  alum  has  come  from  physicians 
who  have  believed  that  the  passage  of  the  alum  into 
the  distribution  pipes  in  the  city,  at  times  when  the 
alkalinity  of  the  water  was  too  low  to  decompose  the 
entire  charge  of  chemicals  being  used,  might  act  in- 
juriously on  the  public  health.  The  only  answers  to 
this  charge  are:  that  there  should  be  no  such  acci- 
dental overdose  in  a  properly  managed  plant,  and 
that  there  are  now  hundreds  of  these  filters  in  use  for 
small  municipal  supplies  where  the  charge  of  the 
chemicals  is  not  carefully  watched,  and  yet  there  is 
not  recorded  a  single  instance  where  it  is  proven  that 
the  health-tone  of  the  community  has  been  lowered 
by  the  use  of  water  filtered  with  the  aid  of  alum. 

Quantity  of  Coagulant  Required. — The  efficiency  of 
the  process  of  rapid  sand-filtration  depends  upon  the 
quantity  of  coagulant  used,  the  time  of  its  application 
to  the  water,  the  composition  of  the  water,  the 
amount  of  subsidence  allowed,  the  thickness  of  the 
sand  layer  in  the  filter,  the  size  of  the  sand  grains,  the 
rate  at  which  the  filter  is  operated,  the  loss  of  head 
allowed  at  the  filters,  the  manner  of  washing  the  fil- 
ters and  of  operating  them,  and  the  care  and  over- 
sight exercised  at  all  times  over  all  stages  of  the 
process. 

The  effect  of  using  too  small  a  quantity  of  coagu- 
lant will  be  a  low  efficiency  in  the  removal  of  ba£- 


204  WATER   F1LTFATION    WORKS. 

teria,  turbidity  and  color.  The  quantity  of  coagulant 
used  should  always  be  less  than  corresponds  to  the 
alkalinity  of  the  water;  in  other  words,  if  there  is  not 
enough  carbonate  of  lime  or  magnesia  in  the  water 
to  neutralize  the  sulphuric  acid  set  free  by  the  chem- 
ical reactions,  the  acidulated  water  will  attack  the  iron 
and  lead  pipes,  in  the  distribution  system,  and  may 
cause  a  great  deal  of  trouble.  If  there  is  not  enough 
lime  in  the  water,  at  most  seasons  of  the  year,  to  per- 
mit sufficient  coagulant  to  be  used,  this  process  will 
generally  not  be  suited  for  its  purification,  as  the  ex- 
pense of  continuously  adding  lime  or  soda-ash,  to- 
gether with  the  cost  of  the  sulphate  of  alumina  treat- 
ment, would  probably  be  higher  than  the  cost  of  other 
processes  of  purification. 

Where  the  water  is  ordinarily  of  proper  composi- 
tion, but  may  be  deficient  in  alkalinity  during  heavy 
floods,  lime  or  soda  may  then,  in  some  cases,  be  sup- 
plied before  adding  the  alum  solution.  The  waters 
of  rivers  and  streams  generally  contain  much  more 
dissolved  alkaline  constituents  per  unit  of  volume  in 
dry  weather  than  during  floods  and  periods  of  full 
flow.  The  quantity  of  coagulant  must,  therefore, 
generally  be  more  carefully  watched  during  high-wa- 
ter periods  than  during  dry-weather  flow.  As  has 
been  already  explained,  small  upland  rivers  generally 
contain  more  suspended  matter,  per  unit  of  volume  of 
flow,  during  floods  than  during  dry  weather,  but  in 
the  case  of  large  lowland  rivers  the  turbidity  is  often 
more  difficult  to  remove  during  dry-weather  flow 
than  during  floods;  each  case  must,  therefore,  b$ 


THEORY  OF  RAPID   SAND-FILTRATION.        2O$ 

studied  by  itself,  and  the  treatment  must  vary  in  ac- 
cordance with  the  conditions  and  requirements. 

The  neutralization  of  the  acid,  set  free  by  the  de- 
composition of  the  sulphate  of  alumina,  changes  the 
dissolved  carbonates  of  lime  and  magnesia  to  the  sul- 
phates of  the  same  bases;  in  other  words,  the  hard- 
ness is  changed  from  temporary  to  permanent; 
generally  with  the  small  quantities  of  chemicals  re- 
quired for  the  treatment  of  water  by  this  process,  the 
change  of  a  portion  of  the  hardness  from  temporary 
to  permanent  will  not  be  a  serious  matter. 

Mr.  Allen  Hazen,  in  reporting  on  the  filtration  of 
the  Pittsburgh  Water-supply,*  places  the  limit  of  the 
amount  of  alum  that  may  safely  be  used,  at  ordinary 
periods  of  flow,  at  three  fourths  the  amount  corre- 
sponding to  the  lime  in  the  water,  allowing  this 
quantity  to  be  increased  about  25  per  cent,  during 
periods  of  high  turbidity.  This  increase  is  per- 
missible, owing  to  the  ability  of  such  waters  to 
receive  a  certain  amount  of  chemical  without  pro- 
ducing coagulation,  as  noted  also  by  Mr.  Fuller  in 
his  Louisville  report. 

The  quantity  of  alum  required  will  depend,  there- 
fore, upon  the  condition  of  the  water  and  the  results 
desired.  In  the  Providence  experiments,  Mr.  Wes- 
ton  found  one  half  grain  per  gallon  of  water  sufficient 
after  the  filter  had  reached  the  stage  of  effective 
operation;  his  method  of  quickly  bringing  the  filter 
to  condition  was  to  charge  it  heavily,  before  starting, 
with  a  dose  of  alum  solution,  equivalent  to  91 1  grains 

*  Report  of  Filtration  Commission,  Pittsburgh,  1899. 


206 


WATER  FILTRATION   WORKS. 


of  sulphate  of  alumina  in  one  pint  of  water,  and  then 
start  the  filter  slowly,  bringing  it  into  effective  opera- 
tion in  about  half  an  hour,  instead  of  from  one  to 
three  hours,  as  required  without  such  dosing.  This 
additional  dose  raised  the  average  charge  to  about 
0.6  grain  per  gallon. 

In  the  Pittsburgh,  Cincinnati  and  Louisville  ex- 
periments the  quantity  of  coagulant  varied  princi- 
pally with  the  degree  of  turbidity  of  the  water.  In 
Cincinnati  and  Louisville  the  problem  of  purification 
resolved  itself  into  securing  an  effluent  without  tur- 
bidity; when  this  was  accomplished  the  bacterial 
efficiency,  and  removal  of  color  and  other  objection- 
able qualities,  was  satisfactory.  So  far  as  is  known, 
turbidity  has  no  direct  effect  on  the  bacterial  effi- 
ciency of  rapid  sand-filters;  that  such  efficiency  is 
greatest  when  turbidity  is  highest  is  accounted  for  by 
the  fact  that  the  particles  of  clay  causing  turbidity  are 
themselves  very  much  smaller  than  the  bacteria,  and 
a  medium  that  will  retain  the  clay  particles  will  not 
allow  the  bacteria  to  pass  through.  The  relation  be- 
tween turbidity  and  quantity  of  chemical  required  at 
Cincinnati,  as  given  by  Mr.  Fuller,  is  shown  in  Table 
XVII. 

TABLE  XVII. 


Turbidity,  Parts 
per  Million. 

Quantity  of  Sul- 
phate of  Aluminum, 
Grains  per  Gallon. 

Turbidity,  Parts 
per  Million. 

Quantity  of  Sul- 
phate of  Aluminum, 
Grains  per  Gallon. 

10 

•  75 

150 

2.65 

25 

1.25 

175 

2.85 

50 

1.50 

2OO 

3-00 

75 

1-95 

300 

3-80 

100 

2.20 

400 

4.40 

125 

2.45 

THEORY  OF  RAPID   SAND-FILTRATION.         20? 

These  quantities  for  the  Cincinnati  conditions 
corresponded  to  an  estimated  average  annual  charge 
of  1.6  grains  per  gallon  of  filtered  water.  During 
freshets  the  optimum  quantity  of  chemical,  according 
to  Mr.  Fuller,  may  deviate  from  the  given  figures  by 
.25  grain.  The  amount  of  chemical  required,  based 
on  three  days  of  preliminary  subsidence  of  the  water, 
he  estimates  at  from  i  to  3  grains  per  gallon  for  most 
days;  occasionally  periods  might  be  expected  when 
as  little  as  0.7  grain  would  suffice,  while  during  other 
periods  much  more  than  3  grains  would  be  necessary. 

The  action  of  the  sulphate  of  alumina  is  not  lim- 
ited, however,  to  the  removal  of  turbidity  and  bac- 
teria; it  possesses  the  property  of  combining  with  the 
coloring  matter  dissolved  in  the  water,  breaking  it 
up,  coagulating  and  precipitating  it  with  the  sus- 
pended matter.  This  property  is  very  useful  in  the 
treatment  of  waters  which  have  acquired  a  dark  color 
from  long  contact  with  peat,  leaves,  grass,  roots  and 
decaying  organic  matter.  Slow  sand-filters,  as  well 
as  those  of  the  rapid  type,  are  almost  powerless  to 
effect  much  change  in  coloring  matter  of  this  kind 
unless  the  water  is  first  treated  with  sulphate  of 
alumina.  The  alum  has  also  the  power  of  uniting  to 
a  certain  extent  with  the  organic  matter  in  solution 
in  the  water,  and  bringing  about  a  higher  chemical 
purification  than  ordinary  slow  sand-filtration,  with- 
out the  alum,  can  accomplish. 

Admission  of  Chemical  Solution  to  the  Water  and 
Time  Necessary  for  Coagulation  and  Secondary  Subsi- 
dence.— In  the  past  the  practice  has  varied  much  in 


WATER  FILTRATION   WORKS. 

regard  to  the  proper  time  and  place  for  the  admix- 
ture of  the  solution  of  aluminum  sulphate.  Some 
plants  were  arranged  so  that  the  solution  passed  into 
the  water  as  it  reached  the  filters,  while  in  others 
some  time  was  allowed  to  elapse  between  the  admis- 
sion of  the  alum  and  the  filtering  of  the  water.  The 
practice  must  necessarily  vary  in  different  works,  be- 
cause the  object  of  coagulation  is  two-fold:  to  reduce 
the  amount  of  suspended  matter  before  it  reaches 
the  filters,  and  to  catch,  in  the  filter,  that  which  can- 
not be  economically  removed  by  subsidence.  With 
turbid  waters,  therefore,  an  economical  solution  of 
the  problem  would  be  obtained  by  finding  that  de- 
sign for  the  works  in  which  the  combined  cost  of 
sedimentation,  coagulation  and  filtration  would  be  a 
minimum.  It  is  obviously  a  waste  of  money  to  apply 
a  coagulant  to  a  water  which  contains  particles  large 
enough  and  heavy  enough  to  settle  out  by  them- 
selves in  a  reasonable  length  of  time — say  24  hours 
or  less.  Obviously  it  also  would  be  a  waste  of  money 
to  apply  a  chemical  for  the  settlement  of  water  con- 
taining a  high  degree  of  turbidity,  partly  of  fine  mat- 
ter and  partly  of  coarse,  until  the  coarser  had  settled 
out  unaided.  As  already  shown  in  Chapter  II.,  rivers 
differ  greatly  in  regard  to  the  character  and  amount 
of  sediment  carried  in  suspension.  The  economical 
period  of  subsidence  must,  therefore,  in  each  case 
be  determined  by  experimental  work.  In  a  great 
many  plants  about  24  hours  has  been  found  to  be 
the  economical  limit  for  the  simple  subsidence  of  the 
greater  part  of  the  suspended  matter,  the  portion 


THEORV  Of  RAPID   SAND-FILTRATION.        20$ 

still  remaining  in  suspension  settling  at  a  very  much 
slower  rate.  A  portion  of  this  matter  still  in  suspen- 
sion, after  24  hours'  subsidence,  will  be  heavy  enough 
to  go  down  in  a  few  hours  when  coagulated  with 
other  particles.  It  is  apparent,  therefore,  that  a  pro- 
cess of  simple  subsidence,  followed  by  coagulation 
and  secondary  subsidence,  will  relieve  the  filters  of 
part  of  the  work,  saving  wash-water,  supervision  and 
attendance.  Another  important  point,  which  was 
discovered  by  Mr.  Fuller  in  Louisville,  is  that  clay 
particles  have  some  faculty  of  absorbing  or  holding 
the  sulphate  of  alumina,  so  that  a  larger  dose  of  the 
chemical  may  be  taken  up  by  the  water  than  is  ac- 
counted for  by  its  alkalinity.  This  is  another  reason 
for  deferring  the  admixture  of  the  chemical  until  after 
the  employment  of  plain  subsidence  to  the  econom- 
ical limit. 

So  far  as  the  removal  of  bacteria  is  concerned, 
with  comparatively  clear  waters,  a  long  period  of 
coagulation  does  not  seem  to  be  advantageous. 
This  was  exemplified  in  Weston's  experiments,  and 
also  in  the  data  given  by  Mr.  Fuller  in  his  Cincinnati 
report.  With  turbid  waters,  however,  time  is  of 
considerable  significance,  a  period  of  from  half  an 
hour  to  six  hours  of  subsidence  greatly  increasing  the 
bacterial  efficiency.  It  is  absolutely  essential  that  the 
chemical  be  applied  continuously,  and  in  the  proper 
proportions,  in  accordance  with  the  changes  in  char- 
acter and  turbidity  of  the  applied  water,  and  in  pro- 
portion to  the  amount  passing  through  the  filter. 
This  is  the  difficult  and  delicate  part  of  the  process. 


2IO  WATER  FILTRATION   WORKS. 

It  requires  on  the  part  of  the  attendants  a  high  de- 
gree of  intelligence  and  a  conscientious  devotion  to 
duty.  A  failure  to  apply  the  chemical  for  a  few  min- 
utes even,  under  some  conditions,  might  be  followed 
by  disastrous  consequences,  which  would,  in  addition 
to  the  actual  inconvenience  and  danger  resulting, 
throw  discredit  on  the  plant. 

Mr.  Fuller  suggests  that  if,  during  the  stage  of 
coagulation  and  subsidence,  the  charge  of  chemical 
be  kept  a  little  below  the  normal,  a  small  additional 
charge  may  be  introduced  as  the  water  enters  the 
filters,  thus  economically  adjusting  the  dose  to  the 
requirements.  This  is  of  importance  in  the  mainte- 
nance of  high  efficiency,  because  with  turbid  waters 
there  is  always  a  tendency  to  a  reduction  of  efficiency 
for  a  few  minutes  after  washing. 

With  clear  waters,  judging  from  the  Providence, 
Pittsburgh  and  Cincinnati  experiments,  it  seems  de- 
sirable to  admit  the  chemical  near  the  filters.  When 
more  than  an  hour  was  allowed  to  elapse  between  the 
time  of  admission  of  the  solution  and  the  passing  of 
the  water  into  the  filters  the  results  did  not  seem  to 
be  so  good. 

Effect  of  Filtering  Medium. — A  coarse  quartz  sand, 
of  uniform  size  of  grain,  is  ordinarily  used  for  rapid 
sand-filters.  As  the  sand  serves  only  the  purpose  of 
arresting  the  coagulated  suspended  matter,  it  may 
be  seen  that  the  finer  the  sand,  within  certain  limits 
of  practicability,  the  thinner  may  be  the  layer,  and 
the  sooner  the  filter  will  reach  a  normal  condition 
in  its  ability  to  remove  turbidity  and  bacteria.  Of 


THEORY  OF  RAPID    SAND-FILTRATION.        211 

course,  if  too  fine,  clogging  will  occur  immediately, 
and  if  too  coarse  too  much  water  will  have  to  be 
wasted  after  putting  the  filters  in  service.  The  sand 
grains  should  be  as  nearly  uniform  in  size  as  possible 
so  that  in  washing  the  bed,  by  reversing  the  current, 
the  particles  of  sand  will  not  be  carried  away  in  the 
wash-water.  Incidentally,  fine  sand  offers  a  greater 
steadying  effect  to  the  flow  of  the  water  than  coarse 
sand,  and,  therefore,  reduces  somewhat  the  proba- 
bility of  breaks  in  the  top-surface  film  and  the  con- 
sequent passage  of  raw  water  through  the  bed.  More 
water  is  necessary,  however,  for  washing  fine  sand 
than  coarse;  it  is  also  probable  that  a  bed  of  fine  sand 
will  require  thorough  sterilization  and  washing  with 
caustic  soda  at  more  frequent  intervals  than  one  of 
coarse  sand.  The  usual  thickness  of  bed  averages 
about  30  inches,  with  coarse  sands;  by  using  rather  a 
fine  river  sand,  however,  Mr.  Fuller  obtained  satis- 
factory results  at  Cincinnati  with  a  depth  of  20  inches. 
Effect  of  Rate  of  Filtration. — Uniform  experience 
indicates  that  the  rate  of  filtration  in  rapid  sand-fil- 
ters operated  by  gravity  (providing  this  rate  is  uni- 
form and  feasible  in  practice)  has  very  little  effect  on 
the  efficiency  of  the  process.  In  filters  of  the  pres- 
sure type,  however,  the  case  is  entirely  different,  be- 
cause in  these  very  great  heads  may  be  suddenly 
thrown  on  the  filters,  causing  the  breaking  through 
of  the  film  and  a  rapid  deterioration  in  the  quality  of 
the  effluent.  This  weakness  of  the  pressure  type  of 
filters,  as  ordinarily  constructed,  is  now  so  well 


212  WATER  FILTRATION   WORKS. 

known  that  they  are  now  rarely  used  for  the  purifica- 
tion of  drinking  waters,  being  replaced  by  the  open, 
or  gravity,  type,  in  which  the  head  cannot  exceed  a 
certain  limit. 

When  the  pressure  filters,  however,  are  located  be- 
tween the  pumps  and  a  large  distributing  reservoir, 
so  that  the  rate  of  filtration  may  be  maintained  quite 
constant,  the  pressure  type  of  filter  may  give  very 
satisfactory  results. 

At  Providence  no  material  difference  in  efficiency 
was  noticeable  with  rates  of  from  116,000,000  to  156,- 
000,000  gallons  per  acre  per  day.  At  Cincinnati  at 
rates  of  from  46,000,000  to  170,000,000  gallons  per 
acre  per  day,  and  at  Pittsburgh,  with  rates  from 
68,000,000  to  146,000,000  gallons,  no  decisive  differ- 
ences in  efficiency  were  apparent.  It  was  very  notice- 
able, however,  in  all  these  experiments,  that  the  num- 
ber of  bacteria  in  the  effluents  fluctuated  with  the 
number  in  the  applied  water.  One  point,  however, 
of  agreement  in  all  recorded  tests,  is  that  the  rate  of 
filtration  should  not  be  allowed  to  change  too  sud- 
denly from  a  low  to  a  high  rate,  as  such  a  procedure 
is  followed  by  the  breaking  loose  and  washing  into 
the  effluent  of  some  of  the  bacteria  and  matter  re- 
tained in  the  filter.  Properly  designed  controllers 
are,  therefore,  necessary  to  prevent  such  fluctuations, 
while  the  filtered  water  should  be  stored  in  a  reservoir 
of  sufficient  capacity  to  balance  the  unequal  rates  of 
draft.  The  proper  capacity  for  filtered-water  reser- 
voirs is  discussed  in  Chapter  VIII. ,  and  the  remarks 
made  on  page  179  regarding  the  proper  height  of  the 


THEORY  OF  RAPID    SAND-FILTRATION.         21$ 

water  surface  relative  to  the  filters  apply  equally  well 
in  the  case  of  rapid  sand-filters. 

It  is  the  universal  experience  that  the  rate  of  fil- 
tration does  not  influence  the  relative  amount  of 
chemical  necessary  for  proper  filtration.  Thus,  if  one 
grain  per  gallon  is  necessary  for  a  rate  of  50,000,000 
gallons  per  acre  per  day,  one  grain  per  gallon  is  suffi- 
cient for  a  rate  of  150,000,000  gallons  per  acre  per 
day. 

Effect  of  Loss  of  Head. — In  operating  a  filter  plant 
as  much  water  should  be  filtered  between  washing 
times  as  possible,  due  regard  being  had  to  economy 
of  operation.  If  washing  is  put  off  too  long,  however, 
the  additional  amount  of  water  passed  at  the  end  of 
the  run,  per  foot  of  head,  will  be  less  than  the  normal. 
Therefore,  the  time  when  washing  should  be  done 
will  be  indicated  when  the  yield  per  foot  of  head 
begins  to  decrease  rapidly  below  the  normal.  This  is 
an  economical  question,  however,  and  does  not  affect 
the  efficiency  of  the  process,  except  indirectly  by 
separating,  as  far  as  possible,  the  periods  of  reduced 
bacterial  efficiency  due  to  washing,  and  thus  to  a 
certain  extent  increasing  the  general  average  effi- 
ciency. 

Mr.  Fuller  found  at  Cincinnati  that  for  rates  of 
120,000,000  gallons  per  acre  per  day,  with  fine  sand, 
the  economical  loss  of  head  was  about  12  feet,  and  he 
concluded  that  "  high  rates  are  more  economical 
than  low  ones,  and  that  the  full  head  which  can  be 
economically  used  efficiently  should  be  provided. 
Just  where  the  economical  limit  of  the  rate  pf  ftltra- 


214  WATER   FILTRATION    WORKS. 

tion  is  can  only  be  determined  from  practical  experi- 
ence with  a  wider  range  of  conditions  than  existed 
here,  but  there  seem  to  be  no  indications  that  the 
capacity  of  a  plant  originally  constructed  on  a  me- 
dium-rate basis  (100,000,000  to  125,000,000  gallons 
per  acre  daily)  could  not  readily  and  economically 
be  increased,  as  the  consumption  demanded,  to  rates 
at  least  as  high  as  the  highest  tried  here  (170,000,000 
gallons  per  acre  daily),  provided  the  full  economical 
increase  in  loss  of  head  could  be  obtained." 

Negative  heads  with  this  process  are  practicable, 
and,  according  to  Mr.  Fuller,  desirable  if  the  section 
of  greatest  resistance  is  located  at  the  throats  of  the 
strainers  instead  of  in  the  sand  layer.  The  liberation 
of  the  dissolved  air,  if  any,  will  then  occur  below  in- 
stead of  in  the  sand  layer  where  clogging  is  taking 
place,  and  it  will  then  not  have  a  tendency  to  reduce 
the  capacity  of  the  filters,  as  has  been  the  case  when 
slow  sand-filters  were  operated  under  negative  heads. 

Effect  of  Washing  Rapid  Sand-filters. — Rapid  sand- 
filters,  after  several  hours  of  service,  gradually  clog 
up  so  that  the  yield  of  filtered  water  begins  to  dimin- 
ish. When  this  time  comes  they  are  washed  by  re- 
versing the  direction  of  the  current  of  water  through 
them,  at  the  same  time  agitating  the  sand  in  such  a 
manner  that  the  dirty  gelatinous  coating  on  the  sur- 
face of  the  filters  and  on  the  grains  of  sand  is  washed 
off  and  carried  away.  It  has  been  observed  that 
after  washing  the  number  of  bacteria  in  the  effluent 
is  considerably  increased,  for  a  period  varying  from 
a  few  minutes  to  about  three  hours.  It  has  generally 


THEORY  OF  RAPID    SAND-FILTRATION.         21  5 

been  the  practice,  therefore,  to  allow  the  first  water 
passing  through  after  washing  to  run  to  waste.  Mr. 
Fuller,  as  the  result  of  his  Louisville  and  Cincinnati 
experiments,  holds  the  opinion  that  where  the  coagu- 
lant is  properly  applied  and  the  washing  is  properly 
done,  it  is  unnecessary,  at  moderate  rates  of  filtra- 
tion, to  waste  any  of  the  water  after  washing,  as  the 
reduction  of  general  efficiency  following  the  dis- 
charge of  this  small  amount  of  water  not  quite  so 
good  as  the  average,  would  not  be  felt  in  a  large 
plant  composed  of  several  units,  only  one  of  which, 
perhaps,  might  be  washed  at  one  time. 

After  filters  have  been  in  service  for  several 
months  their  bacterial  efficiency  generally  runs 
down,  and  even  washing  will  not  restore  them  to 
their  best  condition.  Mr.  Weston  found  at  Provi- 
dence that  after  about  six  months  it  was  necessary 
to  wash  out  the  filters  with  caustic  soda  in  order  to 
place  them  again  in  a  condition  of  normal  efficiency. 

Effect  of  Trailing. — When  filters  have  been  in  ser- 
vice for  several  hours,  and  surface  clogging  has  re- 
duced their  capacity  somewhat,  an  expedient  called 
trailing  is  sometimes  resorted  to.  This  consists  of 
scoring  the  top  surface  of  the  sand  in  concentric  rings 
or  symmetrical  patterns  to  break  the  continuity  of 
the  surface  film,  and  thus  increase  the  yield  of  the 
filter.  The  effect  of  this  treatment,  as  reported  from 
the  Pittsburgh  experiments,  if  the  sand  is  coarse, 
is  to  increase  the  yield,  at  the  expense  of  puri- 
fication, while  if  the  sand  is  fine  the  detrimental 
effect,  in  point  of  purification,  is  less  noticeable.  The 


2l6  WATER  FILTRATION    WORKS. 

effect  on  the  yield  is  not,  however,  always  easy  to 
foretell,  as  with  some  waters  the  particles  of  sus- 
pended matter  may  be  carried  down  so  far  into  the 
filter  that  surface  agitation  will  not  loosen  up  the 
material  enough  to  increase  its  permeability.  Where 
the  particles  are  coarser  and  are,  therefore,  retained 
nearer  the  top,  surface  agitation  may  be  more  effec- 
tive. 


CHAPTER  VI. 

THE  CONSTRUCTION  AND  OPERATION  OF  RAPID 
SAND-FILTERS. 

Up  to  the  present  time  rapid  sand-filter  plants 
have  been  erected  by  one  or  another  of  several  com- 
panies controlling  patents  on  the  processes  and  on 
the  various  parts  of  the  different  makes  of  filters.  The 
fundamental  patent  covering  the  continuous  applica- 
tion of  a  coagulant  in  connection  with  rapid  sand- 
filtration  has  now  expired. 

The  city  of  Louisville,  Ky.,  taking  advantage  of 
the  lapse  of  this  patent,  has  prepared  plans  for  rapid 
sand-filters  of  different  design  from  any  heretofore 
constructed. 

The  various  commercial  types  of  rapid  sand-filters 
differ  from  one  another  principally  in  the  means  used 
for  adding  the  chemical  to  the  water,  in  the  strainer 
system  for  retaining  the  sand,  in  the  manner  of  wash- 
ing the  sand,  in  the  manner  of  regulating  the  rate  of 
filtration,  the  method  of  accomplishing  coagulation, 
and  the  method  of  admitting  the  water  to  the  filters. 

Gravity  and  Pressure  Filters. — Rapid  sand-filters  are 
built  of  two  types,  gravity  and  pressure.  As  has  al- 
ready been  stated,  the  gravity  type  is  now  used  al- 

217 


2l8  WATER  FILTRATION   WORKS. 

most  exclusively  for  water-supply  purification,  the 
pressure  type  being  more  liable  to  derangement  and, 
unless  placed  between  the  pumps  and  the  distributing 
reservoir,  less  reliable  in  point  of  efficiency.  The 
pressure  type  may  always  find  application,  however, 
in  manufacturing  processes  where  the  removal  of  the 
extremely  fine  clay  particles  is  not  essential. 

In  general,  the  filters  are  tanks  of  steel,  iron, 
or  wood,  containing  in  their  bottoms  systems  of  pipes 
for  drawing  off  the  filtered  water.  To  prevent  the 
sand  from  escaping  with  the  water,  strainers  of  brass- 
wire  cloth  of  fine  mesh,  brass  plates  or  cones  bored 
with  small  holes,  or  slotted  plates  or  rosettes,  have 
been  employed.  The  sand  layer  is  usually  from  about 
two  and  a  half  to  three  feet  thick;  the  sand  is  of  rather 
coarse  grain,  quite  uniform  in  size,  and  the  piping  is 
so  arranged  that  the  water  may  be  admitted  to  the 
top  of  the  filter  and  taken  away,  after  filtration,  from 
the  bottom.  Overflows  are  also  provided,  in  the 
gravity  type,  as  well  as  a  connection  by  which  the 
fitered  water  may  be  forced  back  through  the  filter 
for  washing  the  sand.  Arrangements  are  also  made 
to  permit  the  wasting  of  the  water,  upon  placing  the 
filter  in  operation  after  washing,  until  the  surface  film 
has  again  formed.  The  washing  arrangements,  in  the 
gravity  type,  generally  consist  of  arms,  or  rods,  that 
can  be  lowered  down  into  the  sand.  The  rotation  of 
these  arms,  combined  with  the  upward  motion  of  the 
wash-water  through  the  sand,  loosens  up  and  scours 
off  the  deposits  of  dirt  and  coagulant  which  have 
formed  around  and  between  the  sand  grains.  Other 


RAPID    SAND-FILTERS. 

methods  of  washing  are  described  on  subsequent 
pages. 

In  the  plans  for  the  Louisville  plant  the  filters,  in- 
stead of  being  in  small  circular  units,  are  rectangular 
and  comparatively  large  in  area.  They  have  sides  and 
bottoms  of  concrete  instead  of  sheet  metal  or  wood. 
The  bottoms  also,  instead  of  having  a  system  of  pipes 
and  strainers  for  carrying  away  the  filtered  water, 
have  layers  of  brass-wire  cloth  supported  on  a  frame- 
work of  iron  in  such  a  manner  as  to  form  a  hollow 
floor  under  the  whole  filter  area. 

A  perspective  view  showing  the  construction  and 
arrangement  of  a  Jewell  subsidence  gravity  filter  is 
shown  in  Fig.  37.  In  this  type  of  filter  the  in- 
fluent water  enters  the  subsidence  tank  in  a  direction 
tangential  to  the  periphery,  in  order  to  give  a 
rotary  motion  to  the  water  in  the  tank,  by  which  the 
speed  of  coagulation  may  be  hastened.  The  water  is 
admitted  upon  the  surface  of  the  sand  through  the 
central  vertical  pipe,  and  is  drawn  off  after  passing 
through  the  sand  through  the  delivery  valve,  5. 
After  passing  through  the  controller,  7,  it  goes  to  the 
filtered-water  reservoir.  The  filter  is  washed  by 
revolving  the  rakes  or  agitators,  at  the  same  time 
forcing  filtered  water  upward  through  the  strainer 
system,  by  closing  valves  i,  2,  6,  3  and  5,  and 
opening  valve  4.  After  the  washing  has  been  com- 
pleted, valve  4  is  closed  and  valves  i  and  3  are 
opened,  permitting  the  filtered  water  to  run  to  waste 
until  the  filter  is  again  in  the  proper  condition. 

Filters  of  this  type  are  in  use,  among  other  places, 


22O  WATER  FILTRATION    WORKS. 

in  the  recently  completed  plant  at  East  Albany,  N.  Y. 
A  photographic  view  of  the  interior  of  this  plant  is 
given  in  Plate  XV. 


FIG.  37. — SECTIONAL  VIEW  OF  A  JEWELL  SUBSIDENCE  GRAVITY 
FILTER. 

In  the  Continental  filter,  which  is  shown  in  Figs. 
38  and  39,  the  washing  is  done  by  compressed  air  and 


^3   O 

r  * 


SAND-FILTERS. 


223 


wash-water  used  alternately.  Other  features  of  this 
design  are  the  covering  of  the  strainer  system  with  a 
layer  of  gravel,  and  the  distribution  of  the  raw  water 
over  the  surface  of  the  sand  by  a  trough  extending 


FIG.  38. — PLAN  OF  A  CONTINENTAL  GRAVITY  FILTER  WITH 
AIR  WASH. 

around  the  inner  edge  of  the  filter  and  out  over  the 
top  of  the  sand  in  the  shape  of  a  cross,  in  order  to 
distribute  the  water  evenly  over  the  sand  at  a  very 
low  velocity. 

The   New  York  sectional  wash  gravity  filter   is 
shown  in  Fig.  40.     In  this  type  of  filter  the  central 


224 


WATER  FILTRATION   WORKS. 


valve  is  so  designed  that  the  wash-water  may  be 
forced  through  only  one  section  of  the  filtering  sand 
at  a  time.  By  this  means  more  thorough  scouring 
of  the  sand  grains  is  accomplished  than  if  the  whole 
filter  were  washed  at  once.  The  water  is  admitted  to 


FIG.   39. — SECTIONAL   ELEVATION   OF  A  CONTINENTAL  GRAVITY 
FILTER  WITH  AIR  WASH. 

the  filter  through  a  set  of  perforated  pipes  supported 
above  the  sand  level.  In  Fig.  41  is  shown  the  New 
York  sectional  wash  pressure  filter. 

There  are  several  other  makes  of  rapid  sand-filters 
in  use  for  the  filtration  of  municipal  water-supplies, 
but  the  general  principles  underlying  the  design  of 
such  are  sufficiently  well  illustrated  in  the  types  above 
described. 


RAPID   SAND-FILTERS. 


12$ 


Introduction  of  Chemical  Solution. — The  sulphate  of 
alumina  should  be  of  a  high  grade,  as  the  slight 
economy  resulting  from  the  use  of  low  grades  is  not 
warranted  by  experience.  Customers  frequently  buy 
the  sulphate  on  the  basis  of  the  amount  of  AL2O3  it 


FIG.  40.  —  SECTIONAL  VIEW  OF  A  NEW  YORK  SECTIONAL-WASH 
GRAVITY  FILTER. 


contains,  the  usual  proportion  being  about  17^  per 
cent.  Grades  containing  as  low  as  12  per  cent,  have 
been  used  successfully,  but  in  these  the  insoluble 
compounds,  mostly  silicates,  tend  to  foul  the  pipes 
and  orifices,  making  their  cleansing  necessary  more 


226 


WATER  FILTRATION  WORKS. 


frequently  than  if  a  high  grade  were  used.  The  mix- 
ing tanks  are  generally  of  wood,  of  sufficient  capacity 
to  hold  six  hours'  supply  of  the  solution.  The 


requisite  quantity  of  alum  is  placed  in  a  crate  or  box 
near  the  top  of  the  tank,  and  a  small  stream  of  water 
is  sprayed  upon  it,  percolating  down  through  the 
alum  and  falling  into  the  tank.  The  flow  of  the 


RAPID 


stream  can  be  so  regulated  that  by  the  time  the  tank 
is  filled  the  alum  will  all  be  dissolved.  The  solution 
is  kept  in  agitation  by  stirring  with  mechanical  de- 
vices, or  by  compressed  air  forced  up  through  the 
bottom.  Two  tanks  should  be  provided  so  that  the 
solution  may  be  in  preparation  in  one  while  being 
drawn  from  the  other.  The  alum  solution  goes  from 
the  solution  tanks  to  the  measuring  tank,  from  which 
it  in  turn  flows  into  the  filter  inlet  pipe.  A  typical 
measuring  tank,  for  use  with  gravity  filters,  is  shown 
in  Fig.  42.  The  depth  of  the  solution  over  the  ori- 


FIG.  42.— SECTION  OF  A  TYPICAL  ALUM  SOLUTION  MEASURING 
TANK  FOR  A  GRAVITY  FILTER. 

fice  in  the  bottom  of  the  tank  is  kept  uniform  by  pro- 
viding an  overflow  through  which  the  surplus  may 
flow  back  to  the  solution  tanks.  If  the  solution 
flows  to  the  measuring  tank  by  gravity,  the  overflow 
is  pumped  back  to  the  solution  tanks.  The  dose  of 
the  solution  is  varied,  in  accordance  with  the  char- 


228 


WATER  FILTRATION   WORKS. 


acter  of  the  water,  by  changing  the  size  of  the  orifice 
or  by  changing  the  strength  of  the  solution. 

A  type  of  measuring  tank  for  use  with  pressure 
filters  is  shown  in  Fig.  43.  In  operation  this  tank  is 
first  filled  with  a  known  quantity  of  potash  alum. 


GLOBE  VAXVE 
OUTLET 


FIG.  43. — SECTION  OF  AN  ALUM  SOLUTION  TANK  FOR  A  PRESSURE 

FILTER. 

A  small  stream  of  water  is  then  admitted  through  the 
inlet.  The  water  passes  down  through  the  alum, 
dissolving  it,  and  then  up  through  the  outlet 
pipe  and  into  the  influent  pipe  to  the  filter. 
The  pressure  required  to  cause  this  flow  is  about  ^ 
Ib.  per  sq.  inch  above  the  pressure  as  the  water  enters 
the  filter.  This  is  produced  by  throttling  the  influent 
pipe  between  the  two  connections  with  the  tank. 
An  apparatus  is  sometimes  used  in  connection  with 
this  tank,  to  vary  the  dose  of  solution  in  proportion 
to  the  flow  of  water  to  the  filter.  Great  accuracy, 
however,  is  not  claimed  for  such  regulation. 


RAPID   SAND-FILTERS.  229 

In  large  gravity  plants  the  addition  of  the  chemical 
solution  may  conveniently  be  accomplished  (as  pro- 
posed for  the  Little  Falls  plant  for  the  East  Jersey 
Water  Co.)  by  providing  a  plate  in  the  bottom  of 
the  measuring  tank,  the  plate  having  an  orifice  for 
each  filter,  and  each  orifice  having  the  same  area  as 
the  others.  By  having  several  of  these  plates,  the 
orifices  in  each  plate  having  a  different  area  from 
those  in  the  others,  the  dose  of  chemical  may  be 
varied  in  accordance  with  the  character  of  the  water. 
Further  regulation  of  the  dose  may  be  accomplished 
by  having  the  solution  made  up  in  one,  two,  three  or 
four  per  cent,  mixtures,  and  thus  save  multiplication 
of  plates.  If  a  filter  is  out  of  service  an  orifice  is 
closed  and  the  dose  of  chemical  will  thus  always  cor- 
respond with  the  amount  of  water  going  to  the  filters. 

In  place  of  having  a  gravity  feed  the  chemical  so- 
lution is  sometimes  pumped  into  the  influent  pipe.  A 
successful  pump  for  this  purpose  is  illustrated  in  Fig. 
44.  A  small  propeller  wheel  is  mounted  in  a  section 
of  the  influent  pipe,  and  a  bevel-gear  wheel  on  the 
shaft  of  the  propeller  turns  a  small  shaft  which  car- 
ries the  crank  driving  the  pump.  The  pump  is  made 
of  hard  rubber  and  is  mounted  on  the  pipe.  The 
chemical  solution  is  drawn  from  the  solution  tank 
by  the  pump,  and  forced  into  the  influent  pipe.  This 
apparatus  must  work  very  freely  in  order  to  be  suc- 
cessful; in  fact  its  work  should  be  limited  merely  to 
measuring  the  quantity  of  solution.  It  is  also  neces- 
sary to  restrict  the  section  of  the  influent  pipe  so 
that  the  velocity  at  the  propeller  will  be  at  least  six 


230 


WATtLR  FILTRATION   WORKS. 


feet  per  second,  otherwise  the  velocity  head  will  not 
be  sufficient  to  work  the  apparatus.  The  pump 
should  always  be  kept  very  clean,  and  therefore  a 


-HARD  RUBBER  PUMP 

FIG.  44.— SECTION  OF  AN  ALUM  SOLUTION  PUMP  FOR  EITHER  A 
GRAVITY  OR  A  PRESSURE  FILTER. 


high  grade  of  sulphate  of  alumina  is  desirable  when 
this  apparatus  is  to  be  used.  This  system  of  chem- 
ical feed  may  be  used  with  either  the  pressure  or 
gravity  type  of  filter. 

All  the  piping  in  connection  with  the  chemical  feed 


RAPID    SAND-FILTERS. 


231 


should  be  of  brass  or  of  lead.  Lead  pipe  gives  less 
trouble  with  clogging  than  brass,  and  its  length  of 
life  is  also  greater,  the  brass  pipes  lasting  about  ten 
years. 

Regulating  Apparatus. — The  controller  designed  by 
Edmund  B.  Weston,  C.E.,  is  shown  in  Fig.  45  and 
is  described  by  him  as  follows:* 


USPIDER         , 

FIG.  45.— SECTION  OF  WESTON'S  AUTOMATIC  CONTROLLER. 

"The   necessity   of   an  automatic   controller   for 
measuring  the  flow  through  a  filter-bed  and  keeping 

*  The  Norfolk,  Va.,  Filter  Plant.  Paper  by  E.  B.  Weston  read 
before  2Oth  Annual  Conv.  Am.  Water-works  Association,  at  Rich- 
mond, Va.,  May  16,  1900. 


232  WATER  FILTRATION    WORKS. 

it  perfectly  constant  during  the  process  of  filtration,  is 
of  the  utmost  importance. 

"  The  two  principal  reasons  for  the  necessity  of  an 
accurate  controller  are: 

"  ist.  The  exact  quantity  of  water  passing  through 
the  filter-bed  being  known,  the  correct  quantity  of 
sulphate  of  alumina  solution  can  be  accurately  and 
uniformly  applied  to  the  raw  water,  by  gravity  or 
other  means. 

"  2d.  By  keeping  the  flow  of  water  through  the 
filter-bed  perfectly  constant,  scouring  action  in  the 
filter-bed  is  avoided. 

"  The  controller  is  connected  (outside  of  the  filter) 
to  the  draft  or  delivery  pipe  of  the  filter,  from  which 
the  filtered  water  passes  through  butterfly  valves  in 
the  lower  part  of  the  controller.  The  controller  con- 
tains a  float  mounted  on  a  hollow  float-stem,  operat- 
ing in  guides  at  the  top  and  bottom  of  the  controller. 
Beneath  the  float  is  a  deflector,  designed  to  quiet  the 
incoming  water  and  reduce  any  currents,  thereby  giv- 
ing a  smooth  entrance  to  the  discharge  tube,  and  be- 
ing aided  in  this  respect  by  the  flaring  ring  at  the  top 
of  the  discharge  tube.  Mounted  also  upon  the  float- 
stem  at  a  fixed  distance  below  the  float,  so  as  to  be 
maintained  at  a  constant  depth,  is  a  disc  which  is 
turned  with  a  thin  edge  and  sharp  corners  and  of  such 
a  diameter  as  will  give  the  annular  orifice,  between 
the  disc  and  the  walls  of  the  discharge  tube  and  which 
rises  and  falls  with  the  float,  a  predetermined  area 
proportional  to  the  desired  rate  of  discharge.  The 
float  being  mounted  at  a  fixed  distance  from  the  disc, 


RAPID   SAND-FILTERS.  233 

thereby  maintains  a  constant  head  of  water  upon  the 
movable  annular  orifice.  The  inlet  butterfly  valves 
in  the  lower  part  of  the  controller  are  operated  by 
levers  connected  to  the  float,  so  that  the  rise  and  fall 
of  the  latter  tends  to  close  and  open  them. 

"  The  regulation  of  the  flow  of  water  through  the 
filter-bed  may  be  described  as  follows:  With  a  given 
head  of  water  upon  the  surface  of  the  filter-bed  and 
a  free  discharge  from  the  filter,  the  rate  of  discharge 
will  vary  with  the  condition  of  the  filter-bed.  If,  for 
a  given  level  of  water  in  the  controller,  the  head  on 
the  inlet  pipe  be  such  that  more  water  will  pass 
through  the  inlet  butterfly  valves  than  can  be  dis- 
charged through  the  annular  orifice,  the  level  of  the 
water  in  the  controller  will  rise,  and  with  it  the  float, 
which  will  tend  to  close  the  inlet  butterfly  valves  and 
throttle  the  flow  so  that  equilibrium  is  established 
between  the  supply  to  and  the  discharge  from  the 
controller.  If,  on  the  other  hand,  the  head  on  the  in- 
let pipe  be  reduced,  and  consequently  the  flow 
through  the  butterfly  valves,  the  water  level  in  the 
controller  falls  and  the  float  falling  with  it  increases 
the  opening  of  the  valves  and  thus  restores  the 
equilibrium.  Should  the  head  on  the  inlet  pipe  be 
reduced  below  that  determined  by  the  minimum 
limit,  the  water  level  in  the  controller  will  fall  below 
the  minimum  limit,  the  float  will  be  submerged  less, 
and  consequently  the  head  on  the  annular  orifice  and 
discharge  tube  will  be  diminished  below  the  minimum 
desired.  This  will  indicate  a  needed  washing  of  the 
filter-bed,  which  is  manifested  at  Norfolk  by  an  indi- 


234  WATER  FILTRATION    WORKS. 

eating  water  gauge,  that  is  actuated  by  a  float  in  a 
vertical  pipe  which  is  connected  to  the  inlet  pipe  of 
the  controller.  The  rated  capacity  of  discharge  may 
be  adjusted  by  altering  the  depth  of  submergence  of 
the  disc,  or  by  changing  the  area  of  the  annular  ori- 
fice by  substituting  a  disc  of  different  size.  Air  is  ad- 
mitted below  the  disc  through  the  hollow  float-stem, 
which  has  vents  below  the  disc. 

"  Tests  have  been  made  with  this  design  of  con- 
troller under  heads  ranging  from  0.33  to  18  feet,  and 
have  not  shown  any  practical  measurable  variation 
in  the  discharge." 

Washing  Arrangements. — In  many  of  the  existing 
filter  plants  it  is  difficult  to  wash  the  sand  near  the 
bottom  and  between  the  strainers,  and  the  more  or 
less  polluted  water  in  this  unwashed  sand  is  apt  to  af- 
fect the  quality  of  the  effluent.  The  floor  of  the 
Louisville  filters  is  designed  to  correct  this  by  per- 
mitting all  parts  of  the  area  to  be  washed  alike. 

The  stirring  arms  for  the  Louisville  plant  will  be 
mounted  on  a  travelling  platform  suspended  over  the 
beds  on  rollers,  and  capable  of  being  raised  or  low- 
ered or  transported  sidewise.  This  will  permit  one 
apparatus  to  serve  the  filters  of  the  whole  plant. 

In  Plate  XVI  is  shown  the  agitator  used  in  the 
Jewell  filter  at  the  Pittsburgh  experiment  station. 
The  rods  are  pivoted  to  the  arms  in  such  a  manner 
that  when  revolving  in  one  direction  they  will  stand 
vertically  and  stir  up  the  sand.  When  revolved  in 
the  reverse  direction  they  assume  an  inclination  of 
about  60  degrees  from  the  vertical,  so  that  their  ends 


RAPID    SAND-FILTERS. 

rest  upon  the  surface  of  the  sand.    A  short  chain  is 
attached  to  the  end  of  each  rod,  as  may  be  seen  in 

Fig-  37- 

The  agitator  used  in  the  Warren  filter  is  shown 
in  Plate  XVII.  The  stirring  rods  in  this  apparatus 
are  movable  vertically  by  a  hydraulic  lift  supported 
above  the  filter. 

In  large  plants  the  filters  may  be  washed  by  forc- 
ing air  upward  through  the  sand-bed.  The  air  is  de- 
livered at  the  filters  under  a  pressure  of  about  3 
Ibs.  per  square  inch,  by  rotary  blowers.  On  reach- 
ing the  strainers  the  air  expands  and  lifts  the  bed  of 
sand  and  superincumbent  water  sometimes  to  a 
height  of  two  or  three  inches.  The  bubbles  of  air 
carry  the  sand  grains  upward  with  considerable 
force,  rubbing  them  together,  scouring  them  quite 
effectively,  and  floating  them  about  through  the  en- 
tire depth  of  water  above  the  sand.  Mr.  Charles  L. 
Parmelee,  Chief  Engineer  of  the  New  York  Con- 
tinental Jewell  Filtration  Company,  has  seen  grains 
of  sand  thrown  into  the  air  above  the  surface  of  the 
water,  by  bursting  bubbles,  when  the  water  stood  six 
feet  in  depth  over  the  normal  sand  surface. 

Air-washing  has  been  in  use  since  1896  in  pres- 
sure filters,  and  since  1898  in  filters  of  the  gravity 
type,  and  is  said  to  be  as  effective  as  the  agita- 
tion of  the  sand  with  rakes,  combined  with  the  usual 
water-washing  process  by  reversal  of  current. 

In  washing  with  air  the  air  and  wash-water  are 
used  alternately.  If  the  air  and  water  are  used  to- 
gether considerable  sand  will  be  carried  away  with 


238  WATER  FILTRATION    WORKS. 

the  wash-water.  The  amount  of  power  required  for 
air-washing  is  said  to  be  about  the  same  as  for  agita- 
tion with  rakes,  but  the  amount  of  wash-water  re- 
quired is  about  half  as  much  with  air  as  with  the 
rakes. 

The  air  system,  however,  on  account  of  the  ex- 
pense of  installation,  is  not  used  in  small  plants.  Its 
chief  advantage  in  large  plants  comes  from  permit- 
ting the  use  of  rectangular  filters. 

Air-washing  in  large  plants  is  now  considered 
quite  satisfactory.  Its  use  has  been  recommended  in 
the  large  filter  plant,  for  which  plans  were  recently 
prepared,  to  be  erected  at  Little  Falls,  N.  J. 

Cost  of  Rapid  Sand-filters. — Data  on  the  cost  of  ex- 
isting rapid  sand-filter  plants  are  not  valuable  for 
comparisons,  because  of  the  included  values  of  patent 
rights  and  other  expenses  incident  to  the  business  of 
private  companies.  With  the  expiration  of  the  funda- 
mental patent,  however,  it  becomes  a  simple  matter 
to  design  a  plant  and  estimate  its  cost.  In  the  general 
run  of  large  plants,  with  circular  filters,  the  cost  has 
been  at  the  rate  of  probably  about  $500,000  per  acre 
of  filter  surface,  excluding  the  cost  of  buildings,  foun- 
dations, pumping  machinery,  land,  etc.,  or,  in  other 
words,  they  have  been  about  ten  times  as  expensive 
as  covered  slow  sand-filters  of  the  same  area.  As, 
however,  the  rapid  sand-filters  pass  the  water  at  a 
rate  many  times  faster  than  the  slow  filters,  the  rela- 
tive cost  of  construction,  per  unit  of  water  filtered,  is 
really  only  about  from  one  third  to  one  fifth  as  great 
for  the  rapid  as  for  the  slow  sand-filters.  With  im- 


£   ° 

vO     ? 


RAPID    SAND-FILTERS.  241 

provements  in  design,  in  the  matter  of  larger  units, 
steel  and  concrete  construction,  economy  of  space 
and  piping  resulting  from  the  use  of  rectangular 
filters,  improvements  in  sand-washing  devices,  etc., 
the  cost  of  construction  and  of  operation  of  rapid 
sand-filter  plants  may  be  brought  still  much  lower. 
Cheapness  of  installation  and  efficiency,  combined 
with  economy  in  operation,  are  the  greatest  points  to 
attract  favorable  attention.  When  improvements, 
along  the  lines  suggested  above,  have  been  perfected, 
there  is  no  doubt  but  that  rapid  sand-filters  will  find 
a  more  extensive  field  of  application  in  the  future 
for  the  purification  of  drinking  waters. 

When  constructed  in  small  circular  units  they  also 
offer  an  advantage  in  another  direction,  which  was 
first  noted  in  Philadelphia,  in  the  report  of  the 
Mayor's  Expert  Water  Commission.  A  filtered  sup- 
ply from  the  Schuylkill  and  Delaware  Rivers  was 
recommended.  The  Schuylkill,  on  account  of  its 
small  flow,  could  not  be  depended  upon  to  furnish 
sufficient  water  for  supplying  the  entire  district 
through  which  it  passed,  when,  in  the  future,  the 
population  should  increase  beyond  a  certain  limit. 
As  the  filter  plants  in  the  upper  portion  of  the  city 
would  always  have  to  depend  on  this  stream  for  wa- 
ter, it  was  recommended  that  the  one  lowest  down- 
stream be  made  a  rapid  sand-filter  plant  composed 
of  small  units,  so  that  in  the  future,  when  all 
the  water  of  the  river  was  needed  for  the  upper 
plants,  the  lower  site  could  be  abandoned  and  the 
filters  be  moved  over  to  the  Delaware  River.  This 


242  WATER  FILTRATION    WORKS. 

plan  would  entail  less  loss  than  the  abandonment  of 
an  expensive  slow  sand-filter  plant. 

Operation. — The  water  after  having  been  treated 
with  the  coagulant  from  the  supply  tank,  by  means 
of  an  automatic  feed  which  secures  the  delivery  of 
a  quantity  of  alumina  in  proportion  to  the  amount 
of  water  entering  the  filter,  passes  into  the  settling 
basins  where  coagulation  takes  place  and  a  certain 
amount  of  the  suspended  matter  may  be  precipitated. 
An  energetic  agitation  of  the  water,  after  adding  the 
chemical  solution,  materially  hastens  the  process  of 
coagulation,  thereby  permitting  the  use  of  smaller 
settling  basins. 

When  a  filter  is  clean  the  resistance  to  the 
passage  of  the  water  through  the  sand  layer  is  much 
less  than  after  it  has  been  in  service  for  a  while  and 
the  rate  of  filtration,  with  a  constant  head,  would, 
therefore,  gradually  decrease.  In  other  words,  at 
first  it  would  filter  the  water  too  rapidly.  In  order 
to  regulate  this  speed,  automatic  devices,  called  con- 
trollers, have  been  devised.  These  regulate  the 
speed  to  a  predetermined  rate,  thus  making  the  ac- 
tion of  the  filter  regular  and  uniform.  After  the 
available  head  has  been  used  up  the  filter  must  be 
washed.  The  controller  devised  by  Mr.  Edmund  B. 
Weston  has  already  been  described. 

Period  of  Time  Between  Washings. — The  length  of 
time  between  washings,  at  the  Cincinnati  experi- 
mental plant,  with  fine  sand  in  the  filters,  ranged 
from  8  to  24,  and  averaged  15  hours,  while  with  the 
coarse  sand  these  periods  became  6,  36  and  20 


RAPID   SAND-FILTERS.  24$ 

hours,  respectively.  The  general  average  for  several 
plants  of  which  the  author  has  secured  records 
seems  to  be  about  16  hours.  The  coarse  sand  in  the 
Cincinnati  experiments  could  be  washed  in  about  20 
minutes,  while  the  fine  sand  required  30  minutes. 
In  Mr.  Weston's  Providence  experiments  the  aver- 
age time  of  washing  was  about  n  minutes,  and  the 
water  was  wasted  for  30  minutes  after  washing.  At 
the  Pittsburgh  Experiment  Station  the  quality  of  the 
effluent  was  below  the  standard  for  about  20  min- 
utes after  washing,  and  it  was,  therefore,  found  ad- 
visable to  waste  about  2  per  cent,  of  the  filtered 
water. 

Lost  Sand. — A  certain  amount  of  sand,  depending 
upon  the  judgment  of  the  operatives,  the  fineness 
and  uniformity  coefficient  of  the  sand  and  the  veloc- 
ity of  the  wash-water,  will  be  wasted  or  lost  in  wash- 
ing the  filters.  An  allowance  of  about  3  inches  in 
depth,  per  annum,  would  probably  not  be  excessive 
with  such  sands  as  are  commonly  employed.  The 
lower  the  uniformity  coefficient  the  less  will  be  the 
loss  from  this  cause. 

Labor  for  Operation. — The  labor  necessary  for 
operating  a  rapid  sand-filter  plant  is  a  small  part  of 
the  cost  of  operation,  varying  from  12  to  20  per 
cent.,  usually,  of  the  total  cost.  In  a  plant  in  one  of 
our  southern  cities  having  22  pressure-filters,  with 
a  daily  average  capacity  of  3,000,000  gallons,  the 
filters  are  run  by  two  men  at  salaries  of  $60  and  $40, 
respectively,  per  month.  For  a  larger  plant,  say  of 
50,000,000  gallons  daily  capacity,  the  force  would 


244  WATER  FILTRATION   WORKS. 

probably  consist  of  three  shifts  of  engineers  and 
firemen  and  three  shifts  of  laborers  of  four  to 
each  shift.  The  same  force  could  take  care  of  a 
larger  plant.  A  plant  of  100,000,000  gallons  daily 
capacity  could  probably  be  run  with  three  shifts  of 
engineers  and  firemen  and  three  shifts  of  laborers  of 
six  to  the  shift,  providing  the  filters  did  not  require 
washing  oftener  than  once  in  eight  hours,  could  be 
washed  in  30  minutes  to  the  filter  and  were  in 
units  with  a  daily  capacity  of  not  less  than  1,000,000 
gallons  each. 

Wash-water. — The  normal  rate  for  operating  rapid 
sand-filters  is  from  about  100,000,000  to  150,000,000 
gallons  per  acre  daily,  the  filters  being  allowed  to  run 
until  they  become  so  clogged  that  the  allowable  loss 
of  head  is  consumed.  As  already  stated,  they  are 
washed  by  pumping  or  forcing  filtered  water  back 
through  the  underclrains  at  a  rate  from  three  to  four 
times  as  great  as  that  at  which  the  filters  operate 
normally,  at  the  same  time  agitating  the  sand  beds 
with  revolving  arms.  They  are  also  sometimes 
scoured  by  sectional  washing,  or  by  using  com- 
pressed air  and  wash-water  alternately.  The  quan- 
tity of  wash-water  required  will  depend  upon  the 
size  of  the  grains  of  sand,  the  character  of  the 
raw  water  and  the  amount  of  clogging.  At  Cin- 
cinnati, Mr.  Fuller  found  that  from  4  to  9,  and 
averaging  5  per  cent,  of  the  filtered  water  was 
required  for  washing  with  the  fine  sand,  while  from 
2  to  6,  and  averaging  3  per  cent,  was  needed  with 
the  coarse  sand-filters.  In  the  Providence  experi- 


RAPID   SAND-FILTERS.  24$ 

ments  Mr.  Weston  found  that  4.9  per  cent,  of  the 
filtered  water  was  needed  for  washing  the  sand  and 
that  it  was  necessary  to  waste  about  2.9  per  cent,  on 
starting  the  filters  in  operation  after  washing.  Mr. 
Fuller,  in  his  Cincinnati  report,  states  that  with 
proper  manipulation  no  wastage  of  wash-water  was 
necessary,  as  it  could  be  pumped  back  into  the  sub- 
siding reservoir,  where  the  great  bulk  of  the  bac- 
teria and  suspended  matters  would  be  deposited  by 
plain  subsidence  in  less  than  one  day.  He  also  states 
that  although  the  quality  of  the  filtered  water  was 
inferior  to  the  normal  directly  after  washing  the  fil- 
ters, the  evidence  indicated  that  in  a  large  plant  it 
would  not  be  desirable  or  necessary  to  waste  any 
filtered  water. 


CHAPTER  VII. 
CONCLUSIONS. 

General. — Leaving  out  the  question  of  household 
filters,  which  has  no  place  in  a  work  of  this  charac- 
ter, we  are  now  in  a  position  to  summarize  the 
knowledge  thus  far  gained  by  experience  with  the 
purification  of  water  by  filtration  on  a  large  scale. 
Generally  speaking,  there  are  only  two  principles  upon 
which  municipal  filter-plants  have  been  successfully 
designed  in  this  country.  In  one  type  the  water  is 
filtered  slowly,  through  beds  of  sand,  without  the 
use  of  chemicals  to  aid  the  process;  in  the  other  the 
water  is  filtered  rapidly  through  beds  of  sand  after 
a  coagulating  material  has  been  introduced  into 
the  water.  In  slow  sand-filters  the  most  usualrate 
of  filtration  is  about  3  million  gallons  per  acre  per 
day;  in  other  words,  the  water  passes  down  through 
the  sand  in  a  coumn  having  the  full  area  of  the  fil- 
ter and  a  depth  of  about  ten  feet.  In  rapid  sand- 
filters  the  rate  of  filtration  is  from  30  to  50  times  as 
fast.  The  slow  type  is  suited  for  the  purification  of 
polluted  waters  not  too  highly  colored  by  vegeta- 
tion and  not  carrying  too  great  an  amount  of  finely 
divided  suspended  matter.  The  rapid  type  is  more 
suited  for  the  removal  of  turbidity  and  color;  when 
carefully  operated  rapid  sand-filters  can  give  a  very 

high   efficiency,   but   sufficient   experience   has   not 

246 


CONCLUSIONS.  247 

yet  been  had  to  warrant  the  unqualified  statement 
that  they  are  ordinarily  as  safe  as  the  slow  filters,  in 
the  treatment  of  a  sewage-polluted  water.  Operating 
at  high  rates,  a  break  in  the  regularity  of  manage- 
ment would  be  likely  to  cause  a  great  degree  of  de- 
terioration in  the  effluent;  and  further,  filters  of  the 
rapid  type  are  not  suitable  for  the  economical  treat- 
ment of  very  soft  waters. 

There  are  undoubtedly  situations  where  a  combi- 
nation of  slow  and  rapid  sand-filters  would  prove 
economical,  for  instance,  near  the  line  of  latitude 
where  is  it  doubtful  whether  or  not  it  would  be  eco- 
nomical to  cover  the  filters.  In  such  places  a  com- 
bined plant,  if  circumstances  permit,  might  work  out 
satisfactorily.  The  rapid  filters  could  be  relied  upon 
mostly  in  cold  and  the  slow  ones  in  warm  weather, 
each  serving  at  the  period  of  the  year  when  the  con- 
dition of  the  water,  as  to  pollution,  is  best  suited  for 
the  respective  type,  thus  saving  the  cost  of  roofing 
over  the  slow  sand-filters.  The  relative  areas  of  slow 
and  rapid  filters  would  have  to  be  determined  from 
a  special  study  of  the  prevailing  meteorological  con- 
ditions. A  combination  of  slow  and  rapid  sand-fil- 
ters would  not  prove  of  benefit  in  the  clarification  of 
turbid  waters,  because  while  first  passing  through 
the  rapid  filters  the  coagulant  would  abstract  finer 
matter  from  the  water  than  could  be  removed  sub- 
sequently in  the  slow  filters;  but  in  the  case  of  a 
very  highly  polluted  water  double  filtration,  first 
through  rapid,  and  then,  after  aeration,  through  slow 
sand-filters,  the  essential  conditions  for  the  proper, 


248  WATER  FILTRATION   WORKS. 

action  of  the  two  processes  being  always  kept  in 
view,  might  be  preferable  to  double  filtration  through 
slow  sand-filters,  as  has  been  sometimes  recom- 
mended. With  a  turbid,  sewage-polluted  water  condi- 
tions might  sometimes  arise  that  would  make  it  advis- 
able to  use  sedimentation  and  then  slow  sand-filtra- 
tion, finishing  with  rapid  sand-filtration  in  order  to 
remove  the  last  traces  of  turbidity.  Where  the  water 
is  occasionally  too  turbid,  or  contains  too  much  color 
to  be  successfully  treated  by  slow  sand-filtration,  but 
still  ordinarily  is  of  suitable  character  for  treatment 
by  this  process,  it  may  be  necessary,  at  times,  to  pre- 
cede filtration  by  sedimentation,  with,  or,  perhaps, 
without,  the  aid  of  a  coagulant. 

The  essential  condition  for  the  satisfactory  and 
safe  operation  of  the  rapid  type  of  sand-filters  is  that 
the  dose  of  coagulant  be  continuously  and  properly 
applied.  This  necessitates  the  occasional,  or,  in  some 
cases  it  might  be  more  properly  said,  constant  ex- 
amination of  the  raw  water  for  turbidity  and  alka- 
linity. A  failure  to  apply  a  sufficient  amount  of 
chemical  would  be  followed  by  reduced  bacterial 
efficiency,  and  an  overdose  might  be  followed  by  the 
acidulation  of  the  water  with  its  attendant  evils  of 
corrosion  in  the  street-mains  and  service-pipes.  For 
this  reason  it  is  evident  that  the  rapid  type  is  better 
suited  for  large  cities,  where  the  plant  would  be  of 
sufficient  extent  to  afford  the  constant  services  of  a 
competent  chemist,  than  for  cities  too  small  to  afford 
chemical  supervision.  Of  course,  in  some  places, 
where  removal  of  turbidity  is  the  only  object,  the  ser- 
vices of  a  chemist  could  be  dispensed  with  if  stand- 


CONCLUSIONS.  249 

ards  of  turbidity  and  the  accompanying  quantity  of 
chemical  solution  were  once  established;  but  in  waters 
polluted  with  sewage,  and  varying  in  alkalinity  at  dif- 
ferent stages  of  flow  and  periods  of  the  year,  safety 
from  one  extreme  or  the  other  can  only  be  assured 
with  the  services  of  a  chemist. 

On  the  other  hand,  the  slow  sand-filters  do  not 
generally  require  such  careful  attention.  If  the 
regulating  apparatus  is  properly  designed,  so  that 
the  filters  cannot  be  operated  at  too  high  a  rate, 
there  is  little  concerning  the  efficiency  of  the  filters 
that  will  depend  upon  the  faithfulness  of  unskilled 
laborers. 

The  cost  of  installing  a  covered  slow  sand-filter 
plant,  to  filter  a  given  quantity  of  water  daily,  is  from 
three  to  five  times  as  great  as  the  cost  of  a  rapid  sand- 
filter  plant  of  the  same  capacity.  The  annual  cost 
of  operation,  however,  is  about  the  same,  the  cost  of 
the  chemical  solution,  and  greater  allowances  for 
deterioration,  making  up  for  the  lower  interest 
charges  in  the  case  of  rapid  sand-filters. 

A  few  words  concerning  certain  methods  of  water- 
filtration  in  use  in  foreign  countries  seems  to  be 
appropriate  at  this  point. 

Tlie  Anderson  Process. — The  Anderson  process  of 
water-purification,  which  has  found  considerable  ap- 
plication in  Europe,  is  somewhat  akin  in  principle 
to  the  process  of  rapid  sand-filtration.  The  process 
consists  of  the  filtration  of  the  water  through  beds  of 
sand  after  a  coagulant  has  been  introduced  into  the 
water.  This  coagulant  is  ferric  hydrate,  produced 
by  agitating  filings  and  chips  of  iron  in  the  water. 


2 $0  WATER  FILTRATION-   WORKS. 

Some  oi!  the  iron  is  taken  up  in  solution,  and  after- 
ward, on  exposure  to  the  air,  again  passes  out  of  solu- 
tion in  an  insoluble  flocculent  form;  this  is  re- 
moved, together  with  the  other  impurities,  by 
sedimentation  and  filtration.  The  first  large  An- 
derson plant  was  built  at  Antwerp.  Later  plants 
have  been  installed  at  Choisy-le-Roi,  near  Paris,  and 
at  Boulogne-sur-Seine.  The  objects  of  using  the 
coagulant  are  to  remove  turbidity  and  reduce  the 
area  necessary  for  filtration.  The  iron  process 
is  not  of  use  for  removing  the  stain  due  to 
dissolved  peaty  matter,  because  the  iron  will  form 
a  soluble  compound  with  the  organic  constituents 
of  the  coloring  matter.  Aluminum  sulphate  has 
been  found  to  be  the  best  chemical  for  this  purpose. 

Pasteur-Chamber  land  Filters. — The  best-known 
porcelain  or  artificial-stone  filters  are  the  Pasteur 
and  the  Fischer,  or  Worms,  filters.  The  Pasteur  fil- 
tering medium  consists  of  hollow  unglazed  tubes  of 
porcelain  through  which  the  water  is  forced.  The 
grain  of  the  porcelain  is  so  extremely  fine  that  the 
bacteria  are  retained  on  the  surface  of  the  tubes.  A 
Pasteur  plant  of  considerable  size  has  recently  been 
installed  for  the  municipal  supply  of  Darjeeling, 
India. 

Worms  Filter. — The  Fischer,  or  Worms,  filters 
are  similar  in  principle  to  the  Pasteur,  in  that  they 
depend  upon  the  surface  of  the  filtering  medium  for 
the  retention  of  the  bacteria,  without  the  action  of 
a  coagulant  or  of  the  nitrifying  organism.  The 
slabs  of  artificial  stone,  through  which  the  water  is 
passed,  are  made  of  sand,  silicate  of  lime  and  soda, 


o    w 
^   w 


II 


CONCL  US  IONS.  255 

moulded  into  squares  about  3^  feet  on  a  side.  They 
are  placed  in  pairs,  with  their  concave  sides  together, 
and  the  water  filters  through  to  the  inside  space, 
while  the  dirt  is  left  on  the  outside.  They  are 
cleaned  by  forcing  steam  through  them  in  the  re- 
verse direction.  The  largest  cities  using  these  fil- 
ters are  Worms-on-the-Rhine  and  Arad,  Hungary. 
There  are  numerous  other  filters  of  this  type,  but 
so  far  as  the  writer  knows  none  of  them  has  been 
used  for  municipal  supplies.  Both  the  Pasteur  and 
Fischer  types  of  filter  can  produce  good  results  in 
the  filtration  of  waters  of  certain  kinds,  but  it  is  not 
settled  that  they  are  practicable  of  installation  in  the 
United  States,  because  of  cost  of  construction,  cost 
of  operation  and  lack  of  sufficient  experience  with 
them  to  determine,  in  the  treatment  of  our  waters, 

• 

their  durability  and  the  cost  of  replacing  breakage. 
In  Plate  XVIII  is  given  a  view  of  a  Fischer  plate 
ready  to  be  placed  in  position  in  a  filter  at  the  experi- 
ment station  at  Pittsburgh,  Pa.  An  idea  of  the  con- 
struction of  these  plates  may  be  gained  from  Plate 
XIX,  which  shows  two  units  broken  at  the  same 
station. 

Regarding  the  Maignen  filters,  in  which  a  layer 
of  asbestos  is  spread  over  the  surface  of  the  sand, 
it  is  sufficient  to  say  that  at  the  present  time  the 
system  is  not  in  use  in  the  United  States  for  the 
purification  of  a  municipal  water-supply. 

It  is  hoped  that  when  these  different  processes  may 
have  been  tried  on  a  large  scale  in  this  country  the 
results  of  such  trials  may  be  recorded  in  future  edi- 
tions of  this  work, 


CHAPTER  VIII. 
FILTERED-WATER   RESERVOIRS. 

Location. — When*  the  filtered-water  reservoir  is  to 
be  near  the  filters  it  should  be  located  in  a  position 
to  which  the  water  from  all  the  filters  may  be  con- 
ducted with  the  minimum  length  of  piping.  It 
should  also  be  constructed,  as  stated  on  page  179, 
at  such  an  elevation  that  the  highest  water  level  can 
not  limit  the  filtration  head. 

Shape. — Reservoirs  are  usually  made  rectangular 
in  plan  when  the  topography  of  the  ground  does  not 
require  another  shape.  Circular  or  polygonal  reser- 
voirs are  more  difficult  to  roof  than  the  rectangular 
type,  and  hence  are  rarely  chosen  when  the  rect- 
angle is  possible.  While  the  quantity  of  masonry 
in  the  side-walls  in  such  is  less  than  in  any  other 
shape,  the  constructional  details  of  roofed  reservoirs 
may  counterbalance  this  advantage.  An  example  of 
the  covered  circular  type  is  to  be  seen  at  Arnheim, 
where  the  vaulted  roof  is  carried  on  iron  posts  and 
girders. 

The  reservoir  should  be  divided  into  two  or  more 
independent  basins  by  heavy  cross-walls.  Gates  con- 
trolling openings  through  these  cross-walls  will  fur- 

2{56 


FILTERED-WATER   RESERVOIRS. 

nish  communication  between  the  different  basins. 
Each  basin  should  have  an  independent  junction 
with  the  inlet  pipe  for  filtered  water,  with  the 
outlet  pipe,  and  also  with  the  overflow  and  drain 
pipes. 

Circulation. — To  prevent  the  water  in  the  reser- 
voir from  becoming  stagnant,  caused  by  some  of 
it  not  being  able  to  escape  from  the  remote  cor- 
ners, the  expedient  is  sometimes  adopted  of  dividing 
the  reservoir  by  partition  walls  into  a  number  of  nar- 
row parallel  channels,  each  connecting  with  the  next 
at  alternate  ends,  thus  compelling  all  the  water  to 
move  in  a  continuous  direction  toward  the  outlet. 
The  filtered-water  reservoirs  at  the  Lake  Mueggle 
works,  Berlin,  are  built  in  this  way,  and  also  the  large 
reservoir  for  spring-water  at  Frankfort-on-the-Main. 

Capacity. — In  very  many  of  the  European  filter 
plants  the  filtered-water  reservoir  is  too  small  to 
have  any  effect  as  a  balancing  reservoir;  the  com- 
pensation for  hourly  and  daily  fluctuation  being 
made  by  operating  the  filters  at  a  higher  rate  to 
meet  the  demand.  It  is  needless  to  say,  however, 
that  such  a  practice,  unless  properly  understood  and 
carefully  watched,  is  not  conducive  to  high  efficiency. 
With  clear  waters,  nearly  free  from  bacteria  in  their 
natural  state,  the  danger  of  reduced  efficiency  by  in- 
creasing the  rate  abnormally  is  much  less  than  with 
water  more  polluted,  and,  therefore,  with  compara- 
tively pure  waters  the  storage  may  be  less  than  with 
polluted  waters. 

The  capacity  of  the  filtered-water   reservoir  at 


258  WATER  FILTRATION    WORKS. 

Hamburg  is  about  6.2  per  cent.,  and  at  Berlin  (Lake 
Mueggle)  about  5.6  per  cent,  of  the  average  maximum 
daily  draft.  Since  water  even  after  very  careful  fil- 
tration still  contains  a  small  number  of  bacteria 
which  have  passed  through  the  filter,  and  also  a 
small  amount  of  food  matter,  it  is  essential  that  it 
should  be  delivered  to  the  consumer  as  soon  as  possi- 
ble in  order  that  its  quality  may  not  deteriorate  by 
the  growth  of  these  micro-organisms  during  storage. 
This  consideration  indicates  the  desirability  of  a 
small  reservoir.  On  the  other  hand,  if  the  filtered- 
water  reservoir  is  to  provide  the  only  storage,  a  cer- 
tain amount  of  reserve  is  desirable  in  case  of  fires, 
unless  a  separate  supply  is  provided  for  that  purpose, 
or  a  by-pass  arranged  so  that  in  such  an  emergency 
the  unfiltered  water  can  be  turned  directly  into  the 
mains.  This  plan,  where  the  water  is  treated  more 
to  remove  turbidity  or  color  than  specific  bacteria 
of  contagious  diseases,  is  often  feasible.  Some  au- 
thorities contend  that  the  plan  is  safe  in  all  cases, 
because  the  infection  likely  to  arise  from  such  occa- 
sional delivery  of  raw  water  will  be  less  than  the 
total  infection  resulting  from  the  secondary  growth 
of  the  bacteria  in  large  filtered-water  reservoirs. 
This  is  still,  however,  one  of  those  points  which  can 
never  be  definitely  settled  so  far  as  moderately  pol- 
luted waters  are  concerned.  It  is,  however,  very  de- 
sirable to  have  the  storage  sufficient  to  balance  the 
hourly  fluctuations  in  consumption,  and  for  Ameri- 
can conditions  this  will  require  a  reservoir  capable  of 
holding  about  30  per  cent,  of  the  average  daily  draft, 


FILTERED-WATER  RESERVOIRS. 

presuming1  that  the  filters  have  sufficient  area  to  de- 
liver the  maximum  daily  draft  without  exceeding  the 
maximum  rate  of  filtration  established  for  them. 
This  is  about  a  seven  hours'  supply  at  the  average 
daily  rate;  Professor  Burton  recommends  about  ten 
hours'  supply  as  a  minimum,  in  addition  to  a  fire  re- 
serve, expressed  by  the  empirical  formula: 

Minimum  cubic  feet  of  storage  for  fire  re- 
serve should  be  about  200  times  the  square  root 
of  the  number  of  inhabitants  served. 

The  proper  allowance  for  fire  reserve  for  Amer- 
ican conditions  must  be  determined  by  a  special 
study.  Should  there  be  a  large  distributing  reservoir 
in  the  system  it  should  be  taken  into  account  in  the 
designing  of  the  filtered-water  reservoir,  and  the 
combined  capacity  of  the  two  should  be  sufficient,  at 
least,  to  balance  the  daily  and  hourly  fluctuations  of 
draft  and  provide  the  proper  reserve  for  fires  and  for 
accidents  to  the  machinery.  See  also  page  108. 
Frequently,  as  in  the  Berlin  and  Hamburg  plants, 
other  reservoirs  are  provided  in  the  system,  and  the 
capacity  of  the  small  reservoir  at  the  works  need  not 
then  be  over  from  5  to  7  per  cent,  of  the  daily  mean 
supply,  and  the  rest  of  the  storage  may  be  provided 
for  at  other  points. 

The  most  economical  shape  for  a  rectangular  cov- 
ered reservoir,  if  not  subdivided,  is  the  square.  If 
divided  into  two  basins  by  a  partition  wall  costing 
about  as  much  per  foot  run  as  the  side-walls,  the 
length  of  the  short  side  of  each  basin  should  be 
three  fourths  the  length  of  the  long  side.  For  any 


260  WATER  FILTRATION   WORKS. 

other  number  of  basins  the  formulas  given  on  pages 
115  and  117  may  be  used. 

Depth. — Since  the  relative  amounts  and  costs  of 
excavation  and  masonry  vary  with  the  depth  of  a 
reservoir,  it  is  evident  that  there  is  one  depth  which 
will  be  less  expensive  than  any  other.  For  unroofed 
square  reservoirs,  without  partitions,  the  most  desir- 
able depth  can  be  found  approximately,  according  to 
Prof.  Fruhling,*  by  the  following  formula: 


If  divided  by  a  partition  wall  into  two  basins: 
d  = 


In  the  latter  equation  the  length  of  the  sides  of 
each  basin  are  as  3  to  4. 

In  these  formulas: 
Q  =  capacity  of  reservoir  to  high-water  line  in  cubic 

feet. 

d  =  depth  of  water  in  feet. 
r  =  cost  of  excavation  in  cents  per  square  foot  of 

area  of  finished  reservoir. 
j  =  cost  of  floor  of  reservoir  in  cents  per  square 

foot. 
w  =  cost  in  cents  per  square  foot  of  the  land  upon 

which  the  reservoir  is  built. 

*  Handbuch  der  Ingenieurwissenschaften,  m-i-ii. 


FILTERED-WATER  RESERVOIRS.  26  1 

m  =  a  factor  depending  upon  local  conditions, 
being  the  cost  per  cubic  foot  of  masonry  mul- 
tiplied by  the  percentage  of  thickness  of  ma- 
sonry surrounding  walls  to  their  height. 

These  formulas  will  also  serve  for  rectangular 
covered  reservoirs  if  the  value  of  m  is  increased  to 
correspond  with  the  additional  masonry  in  the  piers, 
and  if  s  is  increased  by  the  additional  cost  of  the 
arches  and  earth  covering  over  them  and  the  piers 
supporting  the  roof.  A  few  trials  will  quickly  de- 
termine the  economical  depth. 

If  the  reservoir  is  covered  with  arches  carried  on 
piers  its  bottom  area  must  be  increased  to  compen- 
sate for  their  submerged  volume. 

In  case  the  reservoir  is  at  a  considerable  eleva- 
tion above  the  works  the  economical  depth  is  in- 
fluenced by  the  cost  of  pumping.  If  the  bottom  of 
the  reservoir  is  established  at  the  proper  height  to 
give  the  required  pressure,  the  water-surface  level 
will  fluctuate  with  the  draft,  and  it  is,  therefore,  ap- 
parent that  the  depth  will  affect  the  cost  of  pump- 
ing. The  economical  depth  may  be  found  by  ap- 
proximation from  the  formula,* 

^(r  +  s+w)  qd*k 


for  an  unroofed  square  reservoir,  and 


for  an  unroofed  reservoir  with  one  partition  wall. 

*  Friihling,  Handbuch  der  Jngenieurwissenschaften,  iii-i-n. 


262  WATER  FILTRATION    WORKS. 

Q  =  capacity  of  reservoir  in  cubic  meters. 
d  =  depth  of  water  in  meters. 
r  =  cost  of  excavation  per  sq.  meter. 
s  =•  cost  of  floor  of  reservoir  per  sq.  meter. 
w  =  cost  of  land  per  sq.  meter. 

m  =  a  factor  depending  on  local  conditions,  being 
the  cost  per  cubic  meter  of  masonry  multiplied 
by  the  percentage   of  thickness  of  masonry 
surrounding  walls  to  their  height. 
k  =  the  capitalized  cost  of  both  operating  expenses 
and    fixed    charges    per    kilogram    of   water 
raised  one  meter  high  per  second,  and 
q  =  the  average  quantity  of  water  to  be  pumped  in 

litres  per  second. 

The  value  of  d  can  be  most  easily  found  by  succes- 
sive approximations. 

From  an  analysis  of  the  formula  it  is  easily  seen 
that  the  influence  of  the  depth  upon  the  total  cost, 
considering  also  the  cost  of  pumping,  will  be  greater 
the  smaller  the  capacity  of  the  reservoir  relative  to 
the  daily  delivery  of  the  pumps. 

It  frequently  will  occur  that  the  shape  of  the 
ground  available,  the  price  of  the  land,  the  depth  of 
the  ground-water  below  the  surface  of  the  ground 
or  the  character  of  the  subsoil  will  determine  the 
size  and  depth  of  the  reservoir,  and  the  theoretical 
considerations  for  greatest  economy  will  have  to  be 
modified  to  suit  such  conditions. 

Bottoms. — To  secure  a  water-tight  reservoir  re- 
quires the  greatest  care  in  workmanship  in  all  par- 
ticulars, and  on  such  works  only  experienced  and 


FILTERED-WATER  RESERVOIRS.  263 

competent  workmen  should  be  employed.  Water- 
tight masonry,  in  the  true  sense  of  the  word,  does 
not  exist,  because  all  ordinary  stones,  mortars  and 
bricks  are  permeable,  to  greater  or  lesser  degrees, 
particularly  under  high  pressures.  A  careful  hand- 
ling of  the  materials  will,  however,  reduce  this  leak- 
age to  very  small  limits  under  the  low  heads  usual 
in  reservoirs  of  this  class.  The  modern  practice  in 
the  construction  of  masonry  reservoirs  tends  tow- 
ards concrete,  brick  and  stone  structures  plastered 
inside  with  cement  mortar  to  secure  tightness.  The 
tendency  is  also  to  dispense  with  clay-puddle,  al- 
though its  use  is  quite  common  in  American,  English 
and  some  of  the  continental  works,  notably  Ham- 
burg and  Warsaw. 

It  seems  to  be  the  general  concensus  of  opinion, 
however,  that  for  filtered-water  reservoirs  built  upon 
sites  where  danger  of  contamination  exists  from 
ground-water  a  bed  of  carefully  placed  and  com- 
pactly puddled  clay  is  essential.  The  clay  should 
preferably  be  mixed  with  about  an  equal  volume 
of  gravel  or  gravel  and  sand.  In  masonry  of  this 
class  the  stones  should  be  rather  small,  sound, 
clean  and  have  rough  surfaces.  The  mortar  should 
be  made  from  a  good  strong  Portland  cement 
and  sharp  clean  sand.  All  the  bricks  and  stones 
should  be  moist  when  laid  so  as  not  to  absorb 
the  water  from  the  mortar.  They  should  be 
laid  quickly,  and  all  the  joints  should  be  filled 
completely.  Concrete  and  range-rubble  work  are 
also  used  extensively,  but  in  all  these  forms  the 


264  WATER  FILTRATION  WORKS. 

ability  to  resist  leakage  is  derived  largely  by  a  plas- 
tering coat,  on  the  walls  and  bottom,  of  Portland 
cement  mortar  from  one  eighth  to  three  sixteenths  of 
an  inch  thick  rubbed  down  with  a  wooden  float.  The 
mortar  should  be  mixed  in  the  proportion  of  about 
one  part  of  cement  to  two  parts  of  sand,  and  a  small 
quantity  of  thoroughly  slaked  lime  may  be  added  to 
increase  the  tightness  and  make  it  easier  to  smooth 
down.  Finally,  the  walls  should  receive  a  wash  of 
neat  cement,  applied  with  a  broom  or  with  a  trowel, 
in  the  latter  case  being  polished  with  a  felt  float. 
Polishing  with  a  metal  float  is  not  satisfactory,  be- 
cause the  surface  is  apt  to  check  in  fine  cracks  when 
the  cement  has  set.  Upon  the  bottom  a  very  thin 
layer  of  dry  cement  should  be  strewn  by  experienced 
men.  This  will  adhere  to  the  damp  concrete  bottom 
and  make  a  very  smooth  hard  surface,  from  which 
algae  and  other  low  plant-life  and  deposits  may  be 
readily  washed. 

The  bottoms  of  the  basins  sometimes  require 
especial  care  in  construction  to  make  them  water- 
tight. The  site  chosen  for  the  filtered-water  reser- 
voir for  the  Hamburg  Water-works  is  adjacent  to 
the  old  settling  basins  at  Rothenburgsort,  which 
were  in  service  at  the  time  the  reservoir  was  being 
built.  The  soil  was  very  unstable  and  great  precau- 
tions were  necessary  to  prevent  the  formation  of 
springs  by  the  excavation  for  the  new  reservoir. 
The  excavation  when  finished  and  leveled  off  was 
lined  with  an  8-inch  layer  of  well-compacted  clay, 
upon  which  the  concrete  bottom  of  the  reservoir  was 


F1LTERED-WATER  RESERVOIRS.  265 

laid.  In  the  mass  of  the  concrete,  which  was  20 
inches  thick,  was  embedded  a  system  of  angle-irons 
crossing  at  right  angles,  and  fastened  to  a  heavy 
channel-iron  running  around  the  exterior  edge. 
Upon  this  concrete  floor  the  walls  and  piers  are  built 
which  carry  the  roof. 

Walls. — In  reservoirs  built  on  a  more  or  less  com- 
pressible foundation,  like  clay-puddle,  the  side-walls 
and  pillars  should  be  carried  up  to  a  considerable 
height,  in  order  that  the  settlement  may  cease  be- 
fore laying  the  floor  so  that  there  may  be  no  future 
settlement  due  to  this  cause. 

Where  iron  pipes  pass  through  the  masonry  there 
is  always  difficulty  in  making  tight  joints.  If  there 
is  danger  of  settlement  of  the  walls  the  pipes  should 
be  carried  through  in  openings  considerably  larger 
than  they  require,  and  all  the  settlement  should  be 
allowed  to  take  place  before  the  hole  is  filled  up. 
To  prevent  leakage  along  the  pipes  a  series  of  col- 
lars should  be  made  on  the  part  passing  through  the 
wall,  and  the  hole  should  then  be  filled  in  with  good, 
strong  concrete,  care  being  taken  that  the  mortar  is 
in  perfect  contact  with  the  iron  work  and  the 
masonry  in  all  places. 

The  side-walls  of  covered  reservoirs  are  usually 
straight,  designed  as  retaining  walls  to  resist  the  ex- 
ternal pressure  of  the  earth,  with  buttresses  opposite 
the  points  from  which  arches  spring,  as  in  the  reser- 
voir at  Geneva,  Switzerland.  In  some  of  the  Ger- 
man and  Italian  reservoirs,  however,  they  are  made 
of  an  economical  section  designed  to  take  the  thrust 


266  WATER  FILTRATION  WORKS. 

of  the  arch  in  a  curved  line  to  the  bottom  of  the 
foundation,  as  at  Wiesbaden.  Mr.  Wm.  H.  Lindley, 
C.E.,  of  Frankfort-on-the-Main,  uses  frequently  a 
comparatively  thin  wall  arched  between  heavy  but- 
tresses spaced  several  feet  apart  in  the  length  of  the 
wall.  The  weight  and  load  of  each  pier  supporting 
the  roof  is  distributed  over  a  large  area  of  the  floor 
by  inverted  arches  turned  between  the  bottoms  of  the 
piers. 

Covers. — Filtered-water  reservoirs  should  nearly 
always  be  roofed  over  to  exclude  light,  to  protect 
them  from  contamination  and  from  the  influence  of 
temperature  changes.  The  roofing  may  be  done  in  a 
variety  of  ways.  Groined,  domical  or  cylindrical  arches 
may  be  used  for  covering,  as  explained  in  chapter  IV., 
or  combination  roofs  of  steel  and  concrete,  or  steel 
and  brickwork  may  be  employed  advantageously. 
The  roofs  of  filtered-water  reservoirs  should  be  made 
water-tight  by  a  coating  of  asphalt  and  asbestic  pa- 
per in  two  or  three  courses,  with  properly  broken 
joints,  and  the  water  percolating  down  to  the  top 
should  be  carried  away  in  a  system  of  drain  pipes. 
For  this  reason  cylindrical  arches  are  more  suitable 
for  reservoir-roofs  than  groined  or  domical  struc- 
tures, as  the  drains  for  the  roof  may  be  laid  in  the 
valleys  between  the  vaults.  Light  trussed  roofs  of 
iron  or  wood,  covered  with  tin,  slates  or  asphalt,  may 
sometimes  be  sufficient  where  cheapness  in  first  cost 
is  a  desideratum.  Roofs  of  this  kind  are  satis- 
factory for  excluding  light  and  affording  plenty 
of  ventilation  in  summer,  but  offer  no  consider- 


FlLfERED-WATER  RESERVOIRS. 

able  protection  against  prolonged  severe  cold,  un- 
less means  are  provided  for  heating  the  space 
above  the  water,  by  which  the  cost  will  be  increased 
considerably.  The  Koenigsberg  niters  are  covered 
with  a  trussed  roof. 

There  may  be,  occasionally,  situations  where,  ow- 
ing to  the  constituents  of  the  water,  it  would  be  un- 
suited,  after  nitration,  to  support  vegetal  growths. 
With  such  waters,  possibly,  covering  of  the  filtered- 
water  reservoirs  would  not  be  essential.  This  ques- 
tion is  one  that  is  occupying  considerable  attention 
on  the  part  of  biologists.  In  some  cases,  where 
proper  conditions  exist,  the  omission  of  covers  over 
the  reservoirs  may  assist  in  saving  a  community  con- 
siderable expense.  It  is  a  point  we'll  worth  exami- 
nation. 

If,  instead  of  using  piers  or  columns  to  support  the 
roof,  solid  thin  partition  walls  be  built  up  from  the 
bottom,  dividing  the  reservoir  into  narrow  parallel 
channels,  the  spaces  being  spanned  with  barrel 
arches,  and  each  wall  pierced  with  an  opening  at 
alternate  ends,  the  water  will  be  kept  in  constant 
motion  and  stagnation  will  be  prevented. 

Ventilation. — There  should  be  a  number  of  venti- 
lation shafts  placed  over  the  reservoir  to  balance  the 
air-pressures  due  to  fluctuating  water  levels.  The 
ventilators  should  be  so  constructed  as  to  admit  air, 
but  exclude  mice,  toads,  insects  and  flies.  Light- 
shafts  covered  with  heavy  wire-glass  should  be  built 
at  convenient  points  to  permit  the  cleaning  of  the 


268  WATER  FILTRATION   WORKS. 

reservoir.  They  should  be  covered  with  an  opaque 
lid  of  some  sort  when  the  reservoir  is  not  in  cleaning. 

A  covering  of  earth  two  or  three  feet  thick  should 
be  spread  over  the  top  of  the  masonry  cover  to  pre- 
vent injury  of  the  structure  by  frost  and  to  prevent 
the  water  from  freezing. 

It  is  desirable  that  the  outlet  for  water  from  the 
reservoir  should  be  at  the  opposite  end  of  the  reser- 
voir from  the  inlet,  in  order  that  all  the  water  may 
circulate  through  the  reservoir  to  avoid  stagna- 
tion in  parts  distant  from  the  outlet.  The  neces- 
sary valves  should  also  be  provided  for  draining  the 
reservoir  and  the  different  compartments,  independ- 
ently, in  case  repairs  should  be  necessary  or  cleaning 
desirable.  Capacious  over-flow  pipes  should  likewise 
be  provided.  Under  the  drain  pipe  there  should  be  a 
sump,  into  which  all  sediment  and  dirt  may  be 
pushed  when  cleaning.  A  large  manhole  over  the 
sump  will  be  necessary  for  the  removal  of  this  mat- 
ter, in  buckets,  by  the  workmen. 

Frequently  it  will  also  be  desirable  to  have  an  au- 
tomatic depth-recording  apparatus,  when  the  reser- 
voir is  at  some  distance  from  the  works,  which  will 
indicate  the  state  of  the  water  in  the  reservoir  upon 
a  chart  in  the  office  of  the  works,  or  the  pump- house, 
if  desirable. 


INDEX. 


PAGB 

Aeration  at  Asbury  Park 4 

at  Atlantic  Highlands 4 

effects  of 4,  42 

at  Koenigsberg 4 

Agitator,  Jewell  filter 234 

Warren  filter 237 

Air- washing  for  rapid  sand-filters 237 

Albany,  N.  Y.,  cleaning  settling  basins 71 

cost  of  filters , 177 

covers  for  filters 127 

gravel  layers  in  filters 154 

sand-washers 157 

settling  basins 59 

Algae  growths,  effects  on  filters  at  Poughkeepsie 188 

Antwerp 189 

Allegheny  River,  turbidity  of 34 

Altona,  amount  of  sediment  collected 66 

scraping  filters 186 

settling  basins 46,  50 

Aluminum  sulphate,  action  of 201 

absorption  by  clay 209 

corresponds  to  alkalinity  of  water 204 

effect  on  color 207,  250 

public  health 203 

quality 225 

American  filters 75 

Anaboena 189 

Analysis  of  sand 84 

269 


276  INDEX. 

PAGS 

Anderson  process 26,  249 

Anderson,  Wm vii 

Antwerp,  algae  growths  on  filters 189 

amount  of  sediment  collected 66 

filters 26 

regulating  apparatus 163 

intake  for 28 

settling  basins 45,  50,  61 

studies  of  organisms  in  filters 188 

water-supply 28 

Anklamm vii 

Arad,  Hungary,  Worms  filters 255 

Arnheim  circular  reservoir 256 

Asbury  Park  water-supply 4 

Ashland,  Wis.,  cost  of  filters 178 

filter-plant 124 

Asterionella  in  Brooklyn  water 189 

removal  of 5 

Atlantic  Highlands  water-supply 4 

Automatic  regulating  apparatus  for  slow  sand-filters 168 

Bacteria,  effects  of  sedimentation  on f 6 

in  underdrains 81 

Bacterial  efficiency 8l 

purification 81 

Bailey,  Geo.  I vii,  183 

Baroda  settling  basins 50 

Barus,  Carl 40 

Beer,  M vii 

Berlin,  covers  for  filters 101,  127 

filters,  regulating  apparatus 164 

frequency  of  scraping  filters 186 

filtration  studies 97 

intake  for 30 

loss  of  head 91 

plan  of  Lake  Muggel  filters 114 

rate  of  filtration 93 

sand-washer 157,  197 

quality  of  water 2 

Bertschinger,    A vii 


INDEX.  271 

PAGE 

Boston,  protection  of  water-supply 23 

reduction  of  organisms  in  water-mains 9 

Boulogne-sur-Seine,  Anderson  process 250 

Brooklyn,  asterionella  in  water 5 

Bradford,  cost  of  cleaning  water-mains II 

Brown,  Andrew 40 

Prof.  C.  C 42 

Bristol,  cost  of  cleaning  water-mains II 

Buffalo,  intake  for 24 

Burton,  Prof.  W.  K , .  163,  172 

Chemicals,  use  of,  to  aid  sedimentation 41 

Choisy-le-Roi,  Anderson  process 250 

Chicago,  intake  for 24 

Cincinnati,  O.,  experiment  station vii,  25,  37,  39,  41,  91 

Clark,  H.  W.,  sedimentation 40 

stored  nitrogen  in  slow  sand-filters 87 

Cleaning  water-mains 10,  n 

Cleveland,  intake  for 24 

typhoid  fever  and  rainfall 18 

Coagulant,  composition 201 

effect  on  efficiency 201 

ferric  hydrate 26,  202,  249 

introduction  into  water 225 

quantity  required 203,  205 

measuring-tanks 227,  228 

mixing-tanks 226 

piping  for 231 

pump 230 

quantity  required  at  Cincinnati 206 

Louisville 206 

Pittsburgh 206 

Providence 205 

time  of  admission 207 

objects. 208 

time  necessary  for 207,  209 

Columbus,  O.,  typhoid  fever  and  rainfall 18 

Color,  failure  of  ferric  hydrate  to  remove  it » . . . .  203 

removal  of,  by  alum 204,  207 

Combinations  of  rapid  and  slow  sand-filters 122,  247 


2/2  INDEX. 

PACK 

Conclusions 246 

Continental  gravity  filter 220 

Controller  for  rapid  sand-filters 231 

Covers  for  filters,  Berlin 127 

effect  in  summer 198 

winter 198 

effect  on  frequency  of  scraping 187 

Koenigsberg 101 

protection  from  ice 101,  119,  120 

Crenothrix  in  Lawrence  filters 184 

Damages  for  pollution  of  water-supply 15 

Danbury,  Ct.,  sewage-pollution 13 

Darjeeling,  India,  Pasteur  filter  plant 250 

Deep  wells,  N.  J.,  removal  of  iron 4 

Delaware  River,  filters  at  Torresdale 55 

sediment 38 

Denbeigh,  cost  of  cleaning  water-mains n 

Depth-recording  apparatus  for  reservoirs 268 

Detroit,  pollution  of  water 23,  24 

typhoid  fever 18 

Double  filtration 200 

Dresden,  quality  of  water 2 

Drown,  Thos.  M vi 

Dumfries,  cost  of  cleaning  water-mains n 

Durham,  cost  of  cleaning  water-mains n 

East  Albany  rapid  sand-filter  plant 220 

East  Jersey  Water  Co 229 

Edinburgh  filters 161 

protection  of  water-supply 17 

sand-washers 159 

Elbe,  turbidity  of 37 

Electric-light  plants  for  filters 133 

Exeter,  cost  of  cleaning  water-mains u 

Ferric  hydrate  coagulant 26,  203,  249 

Filtered  water,  deteriorates  in  storage 258 

Filtered-water  reservoir  bottoms 262 

Berlin 257 

capacity 257 


INDEX.  273 

PAGE 

Filtered-water  reservoir,  circulation  in 257 

covers 266 

depth 260 

Hamburg 264 

location 256 

shape 256 

ventilation 267 

walls 265 

Fire-reserve 259 

Fischer  filters 26,  255 

Flad,  Edward vii,  37,  38 

Fouling  of  water-mains 9 

Fowler,  Chas.  E. 188 

Frankfort-on-Main,  covered  reservoir 52,  257 

Frankland,  Grace 79 

P.  F 42,  79 

Freezing,  effect  on  water 12 

Freuhling,  Prof 49,  50,  260,  261 

Fuller,  Geo.  W. 

26,  35,  37,  91,  205,  207,  209,  210,  211,  213,  214,  215,  244,  245 

Garonne  River,  turbidity 35.  37,  64,  65 

Geneva  reservoir 265 

Gill,  Henry  C 164 

Glasgow,  protection  of  water-supply 17 

Gravel  layers,  slow  sand-filters 142,  153 

Gray,  S.  M 19,  55 

Guisborough,  cleaning  water-mains II 

Hague ,  quality  of  water  at 2 

Halifax,  cost  of  cleaning  mains  II 

Hamburg,  filtered-water  reservoir 258,  264 

growths  in  filters 188 

intake 30 

loss  of  head 91 

Mager  scrapers 198 

rate  of  filtration 93 

regulator 165 

settling  basins 49,  50,  53,  56,  66 

Hardness  of  water 305 


274  INDEX. 


PAGE 


Hazen,  Allen vi,  35,  36,  84,  119,  167,  205 

Hemlock  Lake,  protection  of  supply 22 

Hering,  Rudolph 19,  35,  55,  119 

Holman,  M.  L 43 

Hooker,  E.  H 35 

Hudson  River,  turbidit} 35 

Human  faeces,  fertilizer 22 

Hygienic  efficiency,  definition 81 

Ice  on  open  filters 198 

Hamburg 198 

Poughkeepsie 188 

Ilion,  N.  Y.,  regulating  apparatus 163 

Incrustation  in  pipe  lines g 

Intake,  Antwerp 28 

Berlin 30 

Buffalo 24 

Hamburg 30 

Shanghai 29 

St.  Louis 33 

stable  banks  below  floods 30 

above  floods 29 

Zurich 25 

Intermittent  filters 82,  99 

Iron,  removal  by  aeration 4 

Jackson,  D.  D 9 

Janowski 6 

Jewell  filter 219 

Jordan,  Edwin  O 79 

Kemna,  Ad , 188 

Kirkwood,  James  P v,  165 

Knowles,  Morris vii 

Koenigsberg,  covered  filters 101,  267 

regulating  apparatus 163 

water-supply 4 

Lake  St.  Clair,  pollution 23 

Lake  supplies,  protection 23 

Lanark,  cost  of  cleaning  mains II 

Lancaster,  cost  of  cleaning  mains n 


INDEX.  275 

PAGE 

Lawrence,  Mass. ,  clogging  of  filters 184 

Lawrence  experiment  station 81,  83,  85,  87,  90,  92,  95,  97 

scraping  filters 185 

Lead-poisoning 10 

Leeds,  Prof.  Albert  R 5 

Legal  protection  of  water-supplies 13 

Light,  effect  of,  on  sedimentation 40 

Lime,  use  of,  to  aid  sedimentation 41 

used  with  soft  water  before  coagulation 204 

Lindley,  W.  H vii,  52,  72,  169,  266 

Liverpool,  protection  of  supply 17 

London,  regulating  apparatus 161 

settling  basin 47,  50,  61 

Loss  of  head 90 

Berlin 91 

Hamburg 91 

Louisville,  coagulant 205 

coagulant  absorbed  by  clay 209 

experiment  station vi,  25,  37 

plans  for  rapid  sand-filters 217 

quality  of  water 2 

typhoid  fever  and  rainfall 18 

Mager  scraper,  Hamburg 198 

Maignen  filters 26,  255 

Manchester,  protection  of  supply 17 

settling  basins 62 

Mansergh,  James 17 

Mass.  State  Board  of  Health  vi,  40,  79,  84,  92,  96,  97,  144,  146,  184 

McMath,  R.  E 37 

McMillan,  Hon.  James 35 

Mechanical  filters 75 

Merrimac  River,  turbidity 35 

Meyer,  F.  Andreas vii,  166 

Mill  acts 13 

Mississippi  River,  turbidity 34,  36,  37,  38,  64 

Missouri  River,  turbidity 37,  51 

Modified  English  filters 26 

Mud-deposits  in  reservoirs 6 

Munich,  quality  of  water,  c ,, ., 2 


2/6  INDEX. 

rAGB 

Nethe  River 28 

New  Britain,  sewage  pollution 13 

New  Orleans  experiment  station 25 

Newport,  cost  of  cleaning  mains n 

New  York  City,  protection  of  water-supply 14,  17,  18 

N.  Y.  Continental,  Jewell,  Filtration  Co vii,  237 

N.  Y.  sectional-wash  gravity  filter  225 

pressure  filter 226 

Nitrifying  organism 79 

Nyack  filters,  cost 178 

Odors,  removal  of 42 

Ohio  River,  turbidity 34,  39 

Omagh,  cost  of  cleaning  mains n 

Omaha,  Neb.,  settling  basins 56 

Organisms,  growths  in  Antwerp  filters 188 

Hamburg  filters 188 

Osaka,  Japan,  filter-regulating  apparatus 172 

Oswestry,  cost  of  cleaning  mains n 

Pail  system 22 

Parmelee,  Chas.  L 237 

Pasteur  filters 26,  250 

Paterson,  typhoid  fever  and  rainfall 18 

Per  capita  water-consumption 103 

Peter,  M , vii,  169 

Pflugge 97 

Philadelphia,  cost  of  reservoirs 63 

experiment  station vii,  25 

mud-deposits  in  mains 9 

rapid-filter  plant  proposed  on  Schuylkill 241 

regulating  apparatus 170 

water-supply  report 19.  55 

Piefke 97 

Pipes  affected  by  alum  solution 204 

Pittsburgh,  effect  of  trailing 215 

experiment  station vi,  25,  243,  255 

quantity  of  coagulant 205 

quality  of  water 2 

typhoid  fever  and  rainfall 18 


INDEX.  277 

PACK 

Plagge 8 1 

Pollution  of  water-supplies,  damages  for 15 

Detroit 18 

Port  Huron 23 

Potomac  River,  turbidity  of 35 

Poughkeepsie,  algae  growths  on  niters 188 

Proskauer 97 

Protection  of  water-supplies 13 

Boston 23 

Edinburgh 17 

Glasgow 17 

lake  supplies 23 

Liverpool 17 

Manchester «• 17 

New  York 13 

Rochester 22 

Providence  experiment  station vi,  25 

Public  health,  effect  of  alum  on 203 

Purification  of  water  by  natural  agencies 3 

Rainfall  and  typhoid  fever 18 

Rapid  sand-filters,  air-washing 237 

advantages 246 

combinations  with  slow 122,  247 

construction 217 

Continental  gravity 220 

cost 238 

effect  of  filtering  medium 210 

loss  of  head 213 

rate  of  filtration 211 

trailing 215 

washing 214 

essential  to  use  coagulant 248 

gravity  and  pressure 217 

Jewell  gravity 219 

labor  for  operation 243 

lost  sand 243 

Louisville  rectangular 219 

necessity  for  services  of  chemist 248 

New  York  sectional-wash  gravity 225 

pressure.......  226 


278  INDEX. 


Rapid  sand-filters,  operation 242 

patents 217 

period  of  time  between  washings 242 

plant  at  East  Albany 220 

rectangular,  advantages 241 

time  necessary  for  washing 243 

wash-water 237,  244 

washing  arrangements 234 

washing  with  caustic  soda 215 

wasting  filtered  water 215 

Weston's  Automatic  Controller 231 

Rapid  sand-filtration 76,  201 

introduction  of  chemicals «. 225 

Rate  of  filtration,  Berlin 93 

Hamburg 93 

Reading,  Mass. ,  removal  of  iron 4 

Refilling  slow  sand-filters 100 

Regulating  apparatus,  Berlin 164 

Edinburgh 161 

Hamburg 165 

Ilion 163 

Koenigsberg 163 

London 161 

Osaka 172 

Philadelphia 170 

Stralau 161 

Tokio  , 172 

Tome  Institute 171 

Warsaw 168 

Yokohama 162 

Zurich 168 

Reinisch 98 

Reservoirs,  cost • 63 

for  filtered  water 256 

bottoms 262 

capacity 257 

circulation  in 257 

covers 266 

depth 260 

walls , , ..,..,...  265 


WDEX.  279 

PAGE 

Reservoirs,  preparation  of  site , 6,  20 

protection  to  surface  supplies 19 

Richards,  Ellen  H 79 

Rochester,  N.  Y.,  protection  of  supply 22 

Rotterdam,  intake 46 

settling  basins 61 

Rupel  River 28 

Sacramento  River,  turbidity 64 

Sand 146,  149 

effective  size 84 

lost  in  washing  slow  filters 198 

rapid  filters 243 

transportation  to  washers 190 

uniformity  coefficient 84 

Sand-washers 157,  159 

washing 154 

cost 190 

Sandhurst,  Vic.,  lime  as  coagulant 41 

Sanitary  precautions  during  reservoir  construction 2* 

San  Francisco,  typhoid  and  rainfall 18 

Scarborough,  cost  of  cleaning  mains n 

Schuylkill  River 38 

Scraping  slow  sand-filters 97,  98,  180 

Altona 186 

Berlin 186 

cost 183 

effect  of  covers  on 187 

frequency 185 

in  freezing  weather 198 

Lawrence 185 

under  ice,  Hamburg 198 

quantity  of  sand  removed 185 

Zurich 186,  187 

Sedden,  James  A 40 

Sediment,  amount  to  be  expected 34,  36,  64,  65,  66 

removal  of 60,  61 

Sedimentation,  continuous  vs.  intermittent 60,  64,  73 

effect  of  light 40 

on  bacteria 5 


280  INDEX. 


PACK 

Sedimentation,  purification  by 3.  33 

results  of 41 

use  of  chemicals 41,  204 

lime 41 

Seeding  beds 184 

Settling  basins,  arrangements  for  cleaning 56 

capacity 46 

cleaning 66,  68,  71 

construction 53 

cost 62 

of  cleaning 72 

depth 47 

design 45 

form 51 

Hamburg 49,  50,  53,  56,  66 

inlets  and  outlets 52 

length 48 

London 47,  50,  61 

Manchester 62 

Omaha 56 

operation 63,  66,  67 

regulating  apparatus 56,  59 

roofing 61 

Shanghai,  intake 29 

settling  basins 50,  61 

Simpson,  James v,  161 

Slow  sand-filters,  action  of 80 

advantages 246 

automatic  regulators 168 

bottoms 134 

combined  with  rapid 122,  247 

construction 117 

cost 174 

covering 120 

depth n7 

designing 103 

drainage  of  roof 130 

efficiency 80 

excess  area  required 107 

filtering  sand 146,  149 


INDEX.  28l 

PAGE 

Slow  sand-filters,  gravel 142.  153 

location 113 

operation 179 

refilling  after  scraping 100,  199 

shape 115 

stored  nitrogen  and  bacteria 89 

underdrains 139 

ventilation  and  lighting 133 

Slow  sand-filtration 76 

effect  of  compacting  sand. . . , 84 

effect  of  depth  of  sand 86 

water 93 

loss  of  head 90 

rate  of  filtration 93 

influence  of  age  of  filter 96 

character  of  sand 84,  85 

water 82 

theory 77 

Soda-ash 204 

Soft  water,  soda-ash  used  with 204 

lime  used  with 204 

Somersworth,  N.  H.,  covered  filter 129,  146,  153 

Sterilization  by  vegetation 3,  188 

Strainers 26 

Stralau  filter  regulators 161 

Stralsunder,  covered  filters 122 

St.  Louis,  intake  for 33 

settling  basins 40,42,  43,  48,  50,  53>  55,  61,  65,  72 

Stream-pollution,  Danbury,  Ct 13 

New  Britain,  Ct 13 

Waterbury,  Ct 13 

Streams,  permissible  pollution 13 

self-purification 12,  188 

Storage,  effects  on  filtered  water 5,  258,  266 

surface  water 5,  19 

Strohmeyer,  Otto 188 

Surface  film,  slow  filters 97 

Surface  washings,  effect  on  water 17 

Swamps,  drainage  of 21 

Synedra 189 


282  INDEX. 

PACK 

Temperature,  effect  on  sedimentation 40 

slow  sand-filtration 100 

Tidal  streams,  intakes 28 

Tokio,  filters,  regulating  apparatus 172 

Toledo,  O.,  typhoid  fever  and  rainfall 18 

Tome  Institute  filters,  regulating  apparatus 171 

Tramways  for  sand  haulage 134 

Turbidity 34,  36 

and  quantity  of  coagulant  required 206 

Delaware  River 38 

difficulty  of  removing 204 

Elbe  River 37 

Garonne  River 35,  37,  64,  65 

Hudson  River 35 

Merrimac  River 35 

Mississippi  River 34,  36,  37,  38,  64 

Ohio  River 34,  39 

Potomac  River 35 

removal  by  coagulant 204 

Sacramento  River 64 

Schuylkill  River 38 

Typhoid  fever  and  quality  of  water 2 

rainfall 18 

water-supply i 

Ulverston,  cost  of  cleaning  water-mains n 

Underdrains,  bacteria  in Si 

slow  sand-filters 139 

Vicksburg,  settling  basins 50,  64 

Volontat,  M.  R.  de 35,  64 

Waelhem,  intake  for  Antwerp 28 

Warsaw,  regulating  apparatus 168 

Washington,  D.  C vi,  35 

quality  of  water 2 

Waste  reduction 104 

Wasting  effluents,  rapid  sand-filters 215,  245 

slow  sand-filters 100 

Waterbury,  stream-pollution 13 


INDEX.  283 

PACK 

Water  consumption  per  capita 103 

Water-mains,  cleaning 9 

cost  of 10 

incrustation 9 

Water  and  public  health I 

purification  by  natural  agencies 3 

Water-supplies,  damages  for  pollution 15 

legal  protection 13 

provisions  for  betterment 14 

Wells,  deep 4 

Weston,  E.  B 205,  209,  242,  243,  245 

Wheeler,  Wm , vii,  124 

Whipple,  Geo.  C 6,  9,  37,  189 

Whitehaven,  cost  of  cleaning  water-mains n 

Williams,  G.  S 23 

Wilson,  Jos.  M 19,  55 

Winds,  effect  on  sedimentation >. 39 

Winogradsky 79 

Worms  filters 26,  250 

construction  of  plates 255 

regulating  apparatus 172 

Yang-tse-kiang  River,  intake  for  Shanghai 29 

Yokohama  filters,  regulating  apparatus 162 

Zurich,  automatic  regulating  apparatus 168 

filters 138 

intake 25 

rate  of  filtration 94,  101 

scraping  filters 186,  187 


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_ :  1 


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2 


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7 


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8 


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"    Examination  of  Water 12mo,  1  25 

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Wolff's  Windmill  as  a  Prime  Mover 8vo,  3  00 

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LAW. 

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Davis's  Elements  of  Law 8vo,  2  50 

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Allen's  Tables  for  Iron  Analysis 8vo,  3  00 

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Bolland's  Encyclopaedia  of  Founding  Terms 12mo,  3  00 

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"          "       "          "        Supplement 12mo,  250 

Bouvier's  Handbook  on  Oil  Painting 12mo,  2  00 

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*  Reisig's  Guide  to  Piece  Dyeing 8vo,  25  00 

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Baker's  Masonry  Construction 8vo,  5  00 

Beardslee  and  Kent's  Strength  of  Wrought  Iron  8vo,  1  50 

10 


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Johnson's  Materials  of  Construction 8vo,  6  00 

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Merrill's  Stones  for  Building  and  Decoration 8vo,  5  00 

Merriinan's  Mechanics  of  Materials 8vo,  4  00 

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Halsted's  Elements  of  Geometry 8vo,  1  75 

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Johnson's  Curve  Tracing 12mo,  1  00 

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Wood's  Co-ordinate  Geometry . .  .Svo,  2  00 

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Church's  Mechanics  of  Engineering Svo,  6  00 

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Crehore's  Mechanics  of  the  Girder Svo,  5  00 

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Du  Bois's  Mechanics.     Vol.  L,  Kinematics Svo,  3  50 

Vol.  II.,  Statics Svo,  400 

Vol.  III.,  Kinetics Svo,  350 

Fitzgerald's  Boston  Machinist ISmo,  1  00 

Flather's  Dynamometers 12mo,  2  00 

Rope  Driving 12mo,  200 

Hall's  Car  Lubrication 12mo,  1  00 

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Jones's  Machine  Design.     Part  I.,  Kinematics Svo,  1  50 

12 


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Lanza's  Applied  Mechanics 8vo,  7  50 

MacCord's  Kinematics 8vo,  5  00 

Merriman's  Mechanics  of  Materials 8vo,  4  00 

Metcalfe's  Cost  of  Manufactures 8vo,  5  00 

*Michie's  Analytical  Mechanics . 8vo,  4  00 

Richards's  Compressed  Air 12mo,  1  50 

Eobiuson's  Principles  of  Mechanism 8vo,  3  00 

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Thurston's  Friction  and  Lost  Work 8vo,  3  00 

"         The  Animal  as  a  Machine 12mo,  1  00 

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Weisbach's  Hydraulics  and  Hydraulic  Motors.    (Du  Bois.)..8vo,  5  00 
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METALLURGY. 

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Allen's  Tables  for  Iron  Analysis 8vo,  3  00 

Egleston's  Gold  and  Mercury Large  8vo,  7  50 

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Kunhardt's  Ore  Dressing  in  Europe 8vo,  1  50 

Metcalf's  Steel— A  Manual  for  Steel  Users 12mo,  2  00 

O'Driscoll's  Treatment  of  Gold  Ores. 8vo,  2  00 

Thurston's  Iron  and  Steel 8vo,  3  50 

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Wilson's  Cyanide  Processes 12mo,  1  50 

MINERALOGY   AND  MINING. 

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Barriuger's  Minerals  of  Commercial  Value. .  ..Oblong  morocco,  2  50 

Beard's  Ventilation  of  Mines 12mo,  2  50 

Boyd's  Resources  of  South  Western  Virginia 8vo,  3  00 

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Brush  and  Peufield's  Determinative  Mineralogy.   New  Ed.  8vo,  4  00 

13 


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"       Dictionary  of  the  Names  of  Minerals.. 8vo,  3  00 

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Egleston's  Catalogue  of  Minerals  and  Synonyms Svo,  2  50 

Eissler's  Explosives — Nitroglycerine  and  Dynamite Svo,  4  00 

Hussak's  Rock-forming  Minerals.     (Smith.) Small  Svo,  2  00 

Ihlseng's  Manual  of  Mining Svo,  4  00 

Kunhardt's  Ore  Dressing  in  Europe , Svo,  1  50 

O'Driscoll's  Treatment  of  Gold  Ores Svo,  2  00 

*  Penfield's  Record  of  Mineral  Tests Paper,  Svo,  50 

Rosenbtisch's    Microscopical    Physiography   of    Minerals    and 

Rocks.     (Iddings.) Svo,  500 

Sawyer's  Accidents  in  Mines Large  Svo,  7  00 

Stockbridge's  Rocks  and  Soils Svo,  2  50 

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Walke's  Lectures  on  Explosives Svo,  4  00 

Williams's  Lithology Svo,  3  00 

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STEAM  AND  ELECTRICAL  ENGINES,  BOILERS,  Etc. 

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Baldwin's  Steam  Heating  for  Buildings 12mo,  2  50 

Clerk's  Gas  Engine c Small  Svo,  400 

Ford's  Boiler  Making  for  Boiler  Makers ISmo,  1  00 

Hemen way's  Indicator  Practice 12mo,  2  00 

Kneass's  Practice  and  Theory  of  the  Injector Svo,  1  50 

MacCord's  Slide  Valve Svo,  2  00 

Meyer's  Modern  Locomotive  Construction 4to,  10  00 

Peabody  and  Miller's  Steam-boilers Svo,  4  00 

Peabody's  Tables  of  Saturated  Steam Svo,  1  00 

"          Thermodynamics  of  the  Steam  Engine Svo,  5  00 

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Pray's  Twenty  Years  with  the  Indicator Large  Svo,  2  50 

Pupin  and  Osterberg's  Thermodynamics 12mo,  1  25 

14 


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TABLES,  WEIGHTS,  AND  MEASURES. 

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Carpenter's  Heating  and  Ventilating  of  Buildings 8vo,  3  00 

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Mott's  Composition,  Digestibility,  and  Nutritive  Value  of  Food. 

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Ruddiman's  Incompatibilities  in  Prescriptions 8vo,  2  00 

Steel's  Treatise  on  the  Diseases  of  the  Ox 8vo,  6  00 

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Woodhull's  Military  Hygiene 16mo,  1  50 

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